Obesity is a well-known risk factor for type 2 diabetes, but few data exist on the association between weight changes and diabetes risk in non-obese subjects. This study aimed to investigate the effect of weight changes on the incidence of type 2 diabetes in Korea, using 51,405 non-diabetic subjects. Individuals who developed type 2 diabetes were more likely to be older and male, to have high body mass index (BMI), blood pressure, fasting blood glucose, and total cholesterol, to be current smokers and frequent drinkers, to be hypertensive and hyperlipidemic, and to have a family history of diabetes, compared to those without type 2 diabetes. Compared with the consistently non-obese group, there was a higher hazard ratio for incident diabetes (95% confidence interval) in subjects becoming obese [1.49 (1.26-1.77)] and remaining obese [2.56 (2.34-2.81)] after adjustment for confounding factors. Decreased BMI was significantly associated with lower risks for incident diabetes and the trends were more evident in the non-obese group. However, overall there was no significant association of increased BMI with incident diabetes. In conclusion, weight loss was significantly associated with lower risk for diabetes both in non-obese and obese Koreans, but particularly in the non-obese.