We studied the development of proliferative diabetic retinopathy (PDR) in Japanese insulin-dependent diabetes mellitus (IDDM). Subjects were 373 patients who were diagnosed as IDDM between 1951 and 1984, before the age of 30 years, and had no PDR at the first visit to the Diabetes Center. Development of PDR was analyzed by Kaplan-Meier's life-table method in relation to the duration of IDDM. The cumulative incidence of PDR was 20% at 15 years of duration of IDDM, 40% at 19 years, and 70% at 29 years. Female patients (n = 233) developed PDR significantly faster than males (n = 140) (P < 0.002). In both sexes, patients with onset of IDDM at 0-8 years showed significantly slower development than patients with the onset at 9-17 (P < 0.0001) and 18-29 years (P < 0.001). Impact of femaleness on the development was the greatest in patients with age at onset of IDDM at 9-17 years (P < 0.005). Analysis according to the calendar year at onset of IDDM and diabetes duration at the first visit to the Diabetes Center did not show any significant influence on the development of PDR. In conclusion, sex and age at onset of IDDM may be associated with increased risk for the development of PDR in relation to the duration of diabetes in Japanese IDDM.