The ten- to 16-year predictive value of body weight for diabetes mellitus was evaluated prospectively in two groups of women with initially normal results of glucose tolerance tests. The incidence of diabetes in 602 high-risk (previous transient gestational glucose intolerance) participants was significantly higher for overweight subjects (46.7%) than for those of normal weight (25.6%). In 328 concurrently selected negative control subjects (without previous intolerance), the difference was not significant (4.5% v 1.9%). Another definition of diabetes, indicating further deterioration, gave similar results, although high-risk overweight women had disproportionately higher incidence rates. Being overweight had no substantial predictive value for diabetes mellitus during this period unless the additional evidence for a high-risk classification was present. Among such persons at high risk, excess weight also predicted the severity of the subsequent diabetic condition.