To examine how long-term patterns of weight change affect the risk for diabetes, especially non-insulin-dependent diabetes mellitus, the authors examined the relation of weight change over a period of about 10 years (from the baseline examination in 1971-1975 until the first follow-up examination in 1982-1984) to the 9-year incidence of diabetes mellitus (1984-1992) in a national cohort of 8,545 US adults from the National Health and Nutrition Examination Survey Epidemiologic Followup Study. Diabetes incidence was identified from death certificates, hospitalization and nursing home records, and self-report. In this cohort, 487 participants developed diabetes. The hazard ratios were 2.11 (95% confidence interval (CI) 1.40-3.18) for participants who gained 5- < 8 kg, 1.19 (95% CI 0.75-1.89) for participants who gained 8- < 11 kg, 2.57 (95% CI 1.84-3.85) for participants who gained 11- < 20 kg, and 3.85 (95% CI 2.04-7.22) for participants who gained 20 kg or more compared with participants whose weights remained relatively stable. The authors found no evidence that the results differed by age, sex, or race. They estimated that the population attributable risk was 27% for weight increases of 5 kg or more. Results from this study and other recent studies suggest that the increase in body mass index in the United States that occurred during the 1980s may portend an increase in the incidence of non-insulin-dependent diabetes mellitus with important public health consequences in future years.