BACKGROUND: It is known that Korean NIDDM patients are mainly non-obese and have experienced weight loss frequently during the course of the disease. However, there have been few studies about the patterns of treatment and complications according to the weight changes, Our purpose was to determine the characreristics of diabetes in Korea by examining the differences in the clinical features according to the current weight and the weight changes. METHODS: From 308 Korean NIDDM patients. We obtained the data about the weight at the time of maximal obesity and diagnosis of diabetes and measured the current weight and height. We also evaluared the presence of diabetic retinopathy, nephropathy and neuropathy, We designated the patients with BMI 21kg/m and less as the lean group, the patients with BMI 21 to <26kg/m as the middle-range group and the patients with 26kg/m and over as the obese group. RESULTS: At the time of maximal weight, 61.4% of the patients were obese, but 40.3% were obese at diagnosis and only 33.8% were obese at recruitment. In the lean group, C-peptide was low and the frequency of insulin therapy was high. Although there was no statistical significance, diabetic complications were more frequent in the lean group. The percentage of the patients who lost weight (loss of 10% trom the maximal weight) was 65.9% in the lean group, 42.3% in the middle-range group and 32.7% in the obese group. The prevalence of retinopathy and neuropathy were higher in the group with weight loss, although not significantly. CONCLUSION: Of 308 NIDDM patients, 42.2% experienced weight loss before and after the diagnosis and only 33.8% were obese at recruitment. In the lean group, insulin secretory capacity was low and the frequency of insulin therapy was high. Our study showed that the lean group and the patients who have lost weight tended to have higher prevalence of the complications. The mass prospective study about the clinical characteristics according to weight changes in Korean NIDDM patients would be needed.