A 12-year prospective cohort study of 1,213,829 Koreans aged 30-95 years found that the risk of death from any cause had a J-shaped association with body mass index (BMI), with the lowest risk among those with a BMI of 23.0 to 24.9. The risk of death from respiratory causes was higher among those with a lower BMI, while the risk of death from atherosclerotic cardiovascular disease and cancer was higher among those with a higher BMI. The relative risk of death associated with BMI declined with increasing age. Underweight, overweight, and obese men and women had higher rates of death than those with a normal weight. The association of BMI with death varied according to the cause of death and was modified by age, sex, and smoking history. The study also found that the risk of death from respiratory causes was higher in men with a lower BMI and in women with a BMI of 23.0 to 24.9. The risk of death from atherosclerotic cardiovascular disease and cancer increased with higher BMI. The study suggests that BMI cutoffs for defining overweight and obesity should be lower for Asian populations than for Western populations. The findings indicate that obesity contributes to Korea's cancer burden and that the association between BMI and mortality varies by age, sex, and smoking history. The study highlights the importance of considering these factors when assessing the health risks associated with BMI.A 12-year prospective cohort study of 1,213,829 Koreans aged 30-95 years found that the risk of death from any cause had a J-shaped association with body mass index (BMI), with the lowest risk among those with a BMI of 23.0 to 24.9. The risk of death from respiratory causes was higher among those with a lower BMI, while the risk of death from atherosclerotic cardiovascular disease and cancer was higher among those with a higher BMI. The relative risk of death associated with BMI declined with increasing age. Underweight, overweight, and obese men and women had higher rates of death than those with a normal weight. The association of BMI with death varied according to the cause of death and was modified by age, sex, and smoking history. The study also found that the risk of death from respiratory causes was higher in men with a lower BMI and in women with a BMI of 23.0 to 24.9. The risk of death from atherosclerotic cardiovascular disease and cancer increased with higher BMI. The study suggests that BMI cutoffs for defining overweight and obesity should be lower for Asian populations than for Western populations. The findings indicate that obesity contributes to Korea's cancer burden and that the association between BMI and mortality varies by age, sex, and smoking history. The study highlights the importance of considering these factors when assessing the health risks associated with BMI.