A nationwide, stratified population sample of 534 diabetic Swiss men and women, aged 35-54 yr, participated in a study of vascular disease. The study was based on a common protocol, standardized examination procedures, and centralized laboratory methods. Patients were chosen from a pool of diabetic Swiss with diabetes greater than or equal to 1 yr. After selection, the participants were classified into groups according to age at diabetes onset (greater than 30 or less than 30 yr) and insulin treatment status. Several variables thought to be related to retinopathy incidence were analyzed at the initial examinations: onset of diabetes before age 30, duration of disease, fasting plasma glucose, blood pressure, and insulin therapy. Follow-up examination of 358 of 458 survivors, with a diabetes duration that averaged 20 yr, showed retinopathy significantly and independently associated with initial fasting plasma glucose, systolic blood pressure, and insulin use but not with diabetes duration. Lower rates of retinopathy development were observed during the follow-up period in diabetic patients on antihypertensive therapy at the baseline examination, suggesting that not only lower fasting plasma glucose and systolic blood pressure levels but also blood pressure therapy itself decreases the incidence of retinopathy.