Most Korean patients with non-insulin-dependent diabetes mellitus (NIDDM) have been reported not to be obese, and many of them lost weight significantly during the course of their disease. In this regard, a retrospective cohort study was conducted to determine the relationship between body mass index (BMI, kg/m²) and the risk for NIDDM among Koreans. Subjects who had received a medical examination from 1990 to 1991 and who were available for the detection of NIDDM until September 1999 were included. Subjects who initially had diabetes or were diagnosed as diabetic within 1 yr after enrollment were excluded. We reviewed the medical records of final cohort of 2,531 subjects. Follow-up of this cohort revealed 117 cases with diabetes with an incident of 7.8 per 1,000 person-years. Compared with those with BMI less than 23 kg/m², the adjusted relative risks for diabetes mellitus for those with BMI of 23-24.9, 25-26.9, and greater than 27 kg/m² were 0.85 (0.47-1.50), 1.29 (0.72-2.31), and 3.38 (1.22-4.63), respectively, for men p for trend<0.01) and as for 9.14 (1.99-41.8), 7.36 (1.47-36.8), and 14.5 (3.03-69.2), respectively, for women (p for trend<0.01). These data indicate a direct relationship between obesity and the risk for the development of diabetes, emphasizing the importance of weight control for the prevention of NIDDM in Koreans.