Between 1966 and 1985, 72 type I and 75 type II diabetics developed persistent proteinuria. These patients were divided into two groups according to time of onset of proteinuria (1966 to 1975 and 1976 to 1985, respectively). In these groups, we investigated both the quality of antihypertensive treatment and metabolic control as well as the course of nephropathy and life prognosis. Control of hypertension was markedly improved in type I and type II diabetics with later onset of proteinuria (1976 to 1985), compared with patients who developed proteinuria between 1966 and 1975. However, metabolic control was improved only in type I diabetics. Compared with the previous decade, prevalence of renal insufficiency was lowered by one-third in type I and type II diabetics with later onset of proteinuria (1976 to 1985). During the six-year observation period, 32% of type I and 72% of type II diabetics who developed proteinuria between 1966 and 1975 died, whereas all type I and 54% of type II diabetics with later onset of proteinuria (1976 to 1985) survived. The study shows that conservative treatment methods, especially lowering of blood pressure, may improve prognosis for proteinuric type I and type II diabetic patients.