This study examines the relationship between body mass index (BMI) and mortality in a large cohort of Korean men and women. The research is based on data from the Korean Cancer Prevention Study (KCPS), which included 1,213,829 participants aged 30 to 95 years. The study found that the risk of death from any cause had a J-shaped relationship with BMI, meaning that the lowest risk was associated with a BMI of 23.0 to 24.9. The risk of death from respiratory causes was higher among those with lower BMIs, while the risk of death from atherosclerotic cardiovascular disease or cancer was higher among those with higher BMIs. The relative risk of death associated with BMI declined with increasing age. The findings suggest that underweight, overweight, and obese individuals have higher mortality rates compared to those with normal BMI. The association between BMI and mortality varied by cause of death and was influenced by age, sex, and smoking history. The study highlights the importance of maintaining a healthy BMI to reduce the risk of premature death.This study examines the relationship between body mass index (BMI) and mortality in a large cohort of Korean men and women. The research is based on data from the Korean Cancer Prevention Study (KCPS), which included 1,213,829 participants aged 30 to 95 years. The study found that the risk of death from any cause had a J-shaped relationship with BMI, meaning that the lowest risk was associated with a BMI of 23.0 to 24.9. The risk of death from respiratory causes was higher among those with lower BMIs, while the risk of death from atherosclerotic cardiovascular disease or cancer was higher among those with higher BMIs. The relative risk of death associated with BMI declined with increasing age. The findings suggest that underweight, overweight, and obese individuals have higher mortality rates compared to those with normal BMI. The association between BMI and mortality varied by cause of death and was influenced by age, sex, and smoking history. The study highlights the importance of maintaining a healthy BMI to reduce the risk of premature death.