The study investigated the prevalence of incipient renal dysfunction in two cohorts with identical duration of type I diabetes but with childhood or adult onset of the disease. The pattern of glomerular (albumin, alb) and tubular (alpha(1)-microglobulin, alpha(1)-m, and N-acetyl-beta-D-glucosaminidase, NAG) urinary protein excretion was studied in 97 patients with diabetes onset before the age of 16 years and in 53 patients with manifestation of the disease after this age. Diabetes duration was comparable in both groups [9.0 years (1.5-40.0) versus 9.0 (1.0-34.0), p 30 microg/g creatinine), patients with diabetes onset in childhood showed significantly higher excretion of NAG compared to those with diabetes onset after the age of 16. The excretion of both tubular markers (alpha(1)-m and NAG) correlated significantly with HbA(1c)-values in both groups. In multiple regression analysis, tubular proteinuria (alpha(1)-m) and diabetes duration correlated significantly to microabuminuria (multiple R = 0.60, p < 0.001). These data suggest that there is no difference concerning the prevalence of incipient diabetic glomerulopathy between patients with an early or a late onset of diabetes. However, a more frequent impairment of tubular function was found in young patients with diabetes onset in childhood, which might be due to a non-optimal glycemic control in this population.