This study investigates the prevalence, determinants, and associations of loneliness with mental health, health behavior, and healthcare utilization in a large German adult sample (N = 15,010, aged 35-74 years). Loneliness was reported by 10.5% of participants, with higher rates among women, those without partners, and those living alone. Loneliness was associated with increased risks of depression (OR = 1.91), generalized anxiety (OR = 1.21), and suicidal ideation (OR = 1.35), controlling for sociodemographic variables and other sources of distress. Lonely participants also had higher rates of smoking and physician visits. The findings highlight the significant health risks posed by loneliness, particularly in terms of mental health and healthcare utilization.This study investigates the prevalence, determinants, and associations of loneliness with mental health, health behavior, and healthcare utilization in a large German adult sample (N = 15,010, aged 35-74 years). Loneliness was reported by 10.5% of participants, with higher rates among women, those without partners, and those living alone. Loneliness was associated with increased risks of depression (OR = 1.91), generalized anxiety (OR = 1.21), and suicidal ideation (OR = 1.35), controlling for sociodemographic variables and other sources of distress. Lonely participants also had higher rates of smoking and physician visits. The findings highlight the significant health risks posed by loneliness, particularly in terms of mental health and healthcare utilization.