Although previous studies have linked obesity to diabetes, the risks associated with weight gain or changes in body fat distribution have not been fully elucidated. The authors therefore prospectively examined the relations between changes in body weight and body fat distribution (1986-1996) and the subsequent risk of diabetes (1996-2000) among 22,171 men in the Health Professionals Follow-up Study. Weight gain was monotonically related to risk, and for every kilogram of weight gained, risk increased by 7.3%. A gain in abdominal fat was positively associated with risk, independent of the risk associated with weight change. Compared with men who had a stable waist, men who increased waist circumference by 14.6 cm or more had 1.7 (95% confidence interval: 1.0, 2.8) times the risk of diabetes after controlling for weight gain. In contrast, men who lost more than 4.1 cm in hip girth had 1.5 (95% confidence interval: 1.0, 2.3) times the risk of diabetes compared with men with stable hip circumference. Fifty-six percent of the cases of diabetes in this cohort could be attributed to weight gain greater than 7 kg, and 20 percent of the cases could be attributed to a waist gain exceeding 2.5 cm. Our findings underscore the critical importance of maintaining weight and waist to reduce the risk of diabetes.