Social isolation is a significant health risk for older adults, with multiple forms of isolation rarely studied together. This study combines indicators of social isolation into two scales: social disconnectedness (e.g., small social network, infrequent participation in social activities) and perceived isolation (e.g., loneliness, perceived lack of social support). The research examines how these two forms of isolation independently affect physical and mental health in older adults. Results show that both social disconnectedness and perceived isolation are independently associated with lower self-rated physical health. However, the association between social disconnectedness and mental health may operate through the strong relationship between perceived isolation and mental health. The study concludes that health researchers need to consider social disconnectedness and perceived isolation simultaneously. Social isolation is linked to health risks comparable to smoking and obesity. Individuals with few social connections or frequent feelings of loneliness experience higher rates of morbidity and mortality, infection, depression, and cognitive decline. However, less is known about how and why social isolation affects health compared to other health behaviors. Most research on social connectedness and health focuses on one or two measures of social isolation, often due to data limitations. Across various studies, several indicators of isolation have been associated with worse health, such as living alone, having a small social network, low participation in social activities, and feelings of loneliness. A second problem is that disciplinary differences have led to a disjuncture in research on social isolation and health, with psychological research focusing on subjective aspects of isolation and sociological research emphasizing social integration. This leaves many important questions unanswered, such as whether the link between social disconnectedness and health reflects the impact of loneliness or if they separately diminish health. The study uses data from the National Social Life, Health, and Aging Project (NSHAP), a population-based survey of 3,005 community-residing older adults. The study combines various indicators of isolation into two reliable scales capturing social disconnectedness and perceived isolation. Using these scales, the study compares the contributions of social disconnectedness and perceived isolation to physical and mental health. Social isolation has been linked to worse health across all age groups, but the study focuses on older adults. The health risks posed by social isolation may be particularly severe for older adults, especially as they face stressful life course transitions, health problems, and disabilities. Older adults may be uniquely able to optimize social relationships or adjust expectations so that low levels of social connectedness do not precipitate feelings of loneliness or perceived deficits in support. The study finds that social disconnectedness and perceived isolation are independently associated with lower self-rated physical health. However, the association between social disconnectedness and mental health may operate through the strong relationship between perceived isolation and mental health. The study concludes that health researchers need to consider social disconnectedness and perceived isolation simultaneously. The study also finds that perceived isolation may mediate the relationship between social disconnectedness and health. The study suggests that older adults who do not feel isolated at any given level of connectedness fare betterSocial isolation is a significant health risk for older adults, with multiple forms of isolation rarely studied together. This study combines indicators of social isolation into two scales: social disconnectedness (e.g., small social network, infrequent participation in social activities) and perceived isolation (e.g., loneliness, perceived lack of social support). The research examines how these two forms of isolation independently affect physical and mental health in older adults. Results show that both social disconnectedness and perceived isolation are independently associated with lower self-rated physical health. However, the association between social disconnectedness and mental health may operate through the strong relationship between perceived isolation and mental health. The study concludes that health researchers need to consider social disconnectedness and perceived isolation simultaneously. Social isolation is linked to health risks comparable to smoking and obesity. Individuals with few social connections or frequent feelings of loneliness experience higher rates of morbidity and mortality, infection, depression, and cognitive decline. However, less is known about how and why social isolation affects health compared to other health behaviors. Most research on social connectedness and health focuses on one or two measures of social isolation, often due to data limitations. Across various studies, several indicators of isolation have been associated with worse health, such as living alone, having a small social network, low participation in social activities, and feelings of loneliness. A second problem is that disciplinary differences have led to a disjuncture in research on social isolation and health, with psychological research focusing on subjective aspects of isolation and sociological research emphasizing social integration. This leaves many important questions unanswered, such as whether the link between social disconnectedness and health reflects the impact of loneliness or if they separately diminish health. The study uses data from the National Social Life, Health, and Aging Project (NSHAP), a population-based survey of 3,005 community-residing older adults. The study combines various indicators of isolation into two reliable scales capturing social disconnectedness and perceived isolation. Using these scales, the study compares the contributions of social disconnectedness and perceived isolation to physical and mental health. Social isolation has been linked to worse health across all age groups, but the study focuses on older adults. The health risks posed by social isolation may be particularly severe for older adults, especially as they face stressful life course transitions, health problems, and disabilities. Older adults may be uniquely able to optimize social relationships or adjust expectations so that low levels of social connectedness do not precipitate feelings of loneliness or perceived deficits in support. The study finds that social disconnectedness and perceived isolation are independently associated with lower self-rated physical health. However, the association between social disconnectedness and mental health may operate through the strong relationship between perceived isolation and mental health. The study concludes that health researchers need to consider social disconnectedness and perceived isolation simultaneously. The study also finds that perceived isolation may mediate the relationship between social disconnectedness and health. The study suggests that older adults who do not feel isolated at any given level of connectedness fare better