THE PIMA INDIANS HAVE THE WORLD'S HIGHEST reported incidence of diabetes. Since 1965, this population has participated in a longitudinal epidemiological study of diabetes and its complications. The examinations have included a medical history for diabetes and other major health problems. The focus of this study is the correlation between the prevalence of hypertension and glucose tolerance in this population. Of the 4315 adults ages 18 and older, 50% had normal glucose tolerance; 12%, impaired glucose tolerance (IGT); 8%, newly diagnosed diabetes; and 31%, previously diagnosed diabetes of a mean duration of 11 years. Age-sex adjusted prevalence of hypertension was 24% in those with normal glucose tolerance, 34% in those with IGT, and 40% in those with diabetes. Hypertension was more common in men than in women and was positively related to obesity. Of the 2667 children ages 6 to 17 years, 4% had IGT, and 1% had diabetes. Blood pressure was higher in boys than girls and was associated with older age and worse glucose tolerance. Longitudinal analyses of data from 188 children ages 5 to 9 years who had their follow-up exam at ages 18 to 24 revealed no relationship between insulin concentration and blood pressure in either sex. In this group mean blood pressure at followup was positively correlated with relative weight, mean blood pressure, and 2-hour post-load plasma glucose concentration at baseline. In a multiple regression model, relative weight was the strongest predictor of mean blood pressure at the follow-up exam.