The aim of the study was to investigate the influence of diabetes care on the incidence and course of clinical nephropathy in patients with Type 1 (insulin-dependent) diabetes mellitus. After hospitalization at a diabetes care unit within 15 yr after diabetes onset, 246 patients were followed at the care unit, whereas 395 patients were non-attenders. The 2 groups were comparable with regard to age and calendar year at diagnosis, insulin dose and body weight. After 40 yr of diabetes duration the cumulative incidence of persistent proteinuria was significantly lower among patients treated at the unit (31.9% vs 44.8%, p less than 0.003). The median survival after onset of persistent proteinuria was significantly longer among patients followed at the diabetes care unit (p less than 0.003). Among patients not developing diabetic nephropathy, no difference in survival was found between the 2 groups, since 72% of the patients in each group survived 40 yr with Type 1 diabetes. It is concluded that patient care seems to be highly important for the incidence and the course of clinical nephropathy, whereas no such influence could be demonstrated in patients not developing clinical nephropathy.