AIM: To investigate if weight gain during adulthood has effects on the risk of developing impaired glucose tolerance (IGT) or Type 2 diabetes beyond effect of attained weight. RESEARCH DESIGN AND METHODS: Data were obtained from a longitudinal study of two cohorts: one of juvenile-onset obese (n = 248) and one of randomly selected control (n = 320) men, weighed at average ages of 20, 33, 44 and 51 years, respectively. RESULTS: For any given BMI, the risk of IGT was higher the greater the weight gain since age 20 (odds ratio of 1.10 per unit kg/m2 of BMI gain, confidence interval 1.03-1.17, P = 0.004), and weight gain during both the early and later ages contributed to the increased risk. Obese men, maintaining weight since age 20, had lower risk of IGT than non-obese men who became similarly obese by age 51. The risk of Type 2 diabetes increased by weight gain in early adult life, but not by more recent weight gain in the later periods, probably because of the development of Type 2 diabetes leading to weight loss. CONCLUSIONS: Independent of attained level of body weight in middle-aged men, weight gain is associated with increased risk of IGT, and is greater in those not overweight in childhood.