We have studied the prevalence of diabetes and glucose intolerance, both cross-sectionally and longitudinally, in a cohort of 199 offspring of conjugal diabetic parents. Although the prevalence of already known diabetes was low (11.5 per cent), twenty-eight of 123 tested offspring (23 per cent), ranging in age from ten to sixty (mean 32.6) years had latent diabetes on their initial glucose tolerance test. Eighty of 123 tested offspring had a normal initial glucose tolerance test. Forty-one of those whose first glucose tolerance test was normal were retested after a mean of 9.4 years and showed no significant change in mean glucose tolerance. On the basis of questionnaire data only, we estimate that cumulatively 36.5 per cent of offspring will have diabetes by the age of sixty years. However, if all offspring are routinely surveyed with glucose tolerance tests, 60 per cent will have abnormal glucose tolerance by the age of sixty years. In view of the high prevalence of asymptomatic latent diabetes, genetic studies of diabetes should not be based on questionnaire data. In six families all offspring were diabetic; in twenty-one families there was a mixture of diabetic and nondiabetic offspring; and in ten none of the offspring was diabetic. The finding of families with no diabetic offspring suggests the possibility of genetic heterogeneity of diabetes in the parents. Most of the parents had maturity-onset diabetes with a mean age at diagnosis of 54.5 years of age. Diabetes among their offspring was generally of a mild maturity-onset type. Only 2 per cent of offspring at risk had developed juvenile-onset type diabetes. Thus the prevalence of any metabolic abnormalities in offspring of two maturity-onset type parents cannot be assumed to be relevant to the offspring of parents with juvenile-onset type diabetes.