This study evaluates the impact of diabetic nephropathy on the incidence of coronary heart disease, stroke and any cardiovascular disease in the Finnish population, which has a high risk of Type 1 (insulin-dependent) diabetes mellitus and cardiovascular disease. We performed a prospective analysis of the incidence of coronary heart disease, stroke and cardiovascular disease in all Type 1 subjects in the Finnish Type I diabetes mellitus register diagnosed before the age of 18 years between 1 January 1965 and 31 December 1979 nationwide. The effect of age at onset of diabetes, attained age at the end of follow-up, sex, diabetes duration and of the presence of diabetic nephropathy on the risk for cardiovascular disease was evaluated. Cases of nephropathy, coronary heart disease, stroke and all cardiovascular diseases were ascertained from the nationwide Finnish Hospital Discharge Register and National Death Register using computer linkage with the Type I diabetes mellitus register. Of the 5148 Type 1 diabetic patients followed up, 159 had a cardiovascular event of which 58 were coronary heart diseases, 57 stroke events and 44 other heart diseases. There were virtually no cases of cardiovascular disease before 12 years diabetes duration. The cumulative incidence of cardiovascular disease by the age of 40 years was 43% in Type 1 diabetic patients with diabetic nephropathy, compared with 7% in patients without diabetic nephropathy, similarly in men and women. The relative risk for Type 1 diabetic patients with diabetic nephropathy compared with patients without diabetic nephropathy was 10.3 for coronary heart disease, 10.9 for stroke and 10.0 for any cardiovascular disease, similarly in men and women. The presence of nephropathy in Type I diabetic subjects increases not only the risk of coronary heart disease, but also of stroke by tenfold.