OBJECTIVE: To examine the relationship between weight change and duration of overweight and obesity and the incidence of type 2 diabetes in a cohort of middle-aged British men. RESEARCH DESIGN AND METHODS: We carried out a prospective study of cardiovascular disease in men aged 40-59 years at screening (1978-1980), drawn from one general practice in 24 British towns, who completed a postal questionnaire 5 years later (Q5) and for whom data on BMI at year 1 (Q1) and Q5 were available (n = 7,100). Men with diabetes at Q1 or Q5 and men with hyperglycemia at Q1 were excluded from the study (n = 184). The main outcome measure was type 2 diabetes (physician-diagnosed) during a mean follow-up period of 12 years starting at Q5 (1983-1985). RESULTS: In the 6,916 men with no history or evidence of diabetes, there were 237 incident cases of type 2 diabetes during the mean follow-up period of 12 years, a rate of 3.2/1,000 person-years. Substantial weight gain (>10%) was associated with a significant increase in risk of type 2 diabetes compared with that in men with stable weight (relative risk [RR] 1.61 [95% CI 1.01-2.56]) after adjustment for age, initial BMI, and other risk factors. Excluding men who developed diabetes within 4 years after the period of weight change increased the risk further (1.81 [1.09-3.00]). After adjustment and exclusion of men who developed diabetes early in the follow-up, weight loss (> or =4%) was associated with a reduction in the risk of type 2 diabetes, compared with that in the stable group, that reached marginal significance (0.65 [0.42-1.03], P = 0.07). A test for trend that fitted weight change as a continuous covariate showed the risk of diabetes to increase significantly from maximum weight loss to maximum weight gain (P = 0.0009). The lower risk associated with weight loss was seen in obese (> or =28 kg/m2) and nonobese subjects and in men with normal (<6.1 mmol/l) and high (> or =6.1 mmol/l) nonfasting blood glucose levels. Although not statistically significant, this is consistent with a benefit from weight loss. Risk of type 2 diabetes increased progressively and significantly with increasing levels of initial BMI and also with the duration of overweight and obesity (P<0.0001). CONCLUSIONS: This study confirms the critical importance of overweight and obesity, particularly of long duration, in the development of type 2 diabetes. The data support current public health recommendations to reduce the risk of type 2 diabetes by preventing weight gain in middle-aged men who are not overweight and by encouraging weight loss in overweight and obese men.