Duration and degree of metabolic control of diabetes have been known to be major risk factors of the development of diabetic complications, especially diabetic retinopathy. However, the duration from onset of diabetes to puberty was reported to have less influence on the development of retinopathy in young-onset Caucasian IDDM patients. It is uncertain whether this phenomenon is also applicable to young-onset Japanese IDDM patients and if so duration to which age shows less influence on the development of retinopathy. One hundred twenty-three Japanese IDDM patients diagnosed before 25 years of age were recruited for this study in 1983 to investigate the influence of duration and age as the risk factors and determine the critical age at which the influence of duration develops. Subjects had no retinopathy in 1983 and were followed up to the onset of retinopathy or the end of 1993. Statistical analysis of estimates of the duration and metabolic control as the risk factors was performed by Cox's proportional hazard model. The reuslts are as follows; 1. Metabolic control of diabetes significantly influenced the development of background retinopathy in male and female IDDM patients regardless of age at onset. 2. Duration from onset to 12 years of age had less influence on the development of retinopathy in male IDDM patients. 3. For female IDDM patients, duration to 9 years of age had less influence on the development of retinopathy. 4. A remarkably higher incidence of retinopathy was noted in the female IDDM patients with an age of 9 to 14 years at onset than in the female patients over 14 years of age at onset. In conclusion, we suggest that the duration before puberty showed less influence on the development of background retinopathy and the critical age was 12 years for male IDDM patients and 9 years for female IDDM patients.