The frequency of a positive family history of diabetes in diabetic patients has increased in recent studies. In this study, it was 16-33% for type 1 diabetes and 43-49% for type 2 diabetes. It was significantly higher than in non-diabetic subjects, and in type 2 than in type 1 diabetic patients. The prevalence of diabetes in parents and siblings of type 2 diabetic patients was higher than in those of type 1 patients, and it was particularly high in parents of young onset type 2 patients. Among type 2 diabetic patients, positive family history was somewhat lower in those with marked obesity in the past. Comparison of groups with varying degrees of glucose intolerance revealed that a family history of diabetes increased in parallel with the impairment of glucose tolerance. The results suggest that genetic factors in the pathogenesis of diabetes are more important in type 2 than in type 1 diabetes, and in the younger onset and less obese subjects than in older onset and more obese patients for type 2 diabetes.