Not a few patients in Japan with early-onset type 2 (non-insulin-dependent) diabetes become blind due to proliferative diabetic retinopathy (PDR). However, the risk factors are poorly understood. The aim of this study was to determine the risk factors for background diabetic retinopathy (BDR) and PDR by following 394 Japanese patients with early-onset type 2 diabetes diagnosed before 30 years of age (mean age 27, mean blood pressure at entry 116/73 mm Hg). Of the 322 patients who were free of diabetic retinopathy at entry, 88 developed BDR, giving an incidence of 57.7 (95% CI 55.5-60. 0)/1000 person-years. Cox proportional hazard analysis revealed mean HbA(1c) and duration of diabetes to be significant predictors of development of BDR. Of the 160 patients with BDR, i.e., the 72 patients who had BDR at entry and the 88 who developed BDR during the follow-up, 50 developed PDR, giving an incidence of 17.9 (95% CI 13.6-23.6)/1000 person-years. Cox proportional hazard analysis indicated mean HbA(1c) and diastolic blood pressure to be significant predictors of the progression from BDR to PDR. In conclusion, in early-onset Japanese type 2 diabetic patients, the rates of both development of BDR and of progression from BDR to PDR appear to be potentially high. Not only lifetime exposure to glycemia but also a slightly elevated blood pressure level is an important risk factor for progression to PDR.