This report describes the design and baseline demographic and clinical data in the study of the early natural history of diabetic nephropathy (DN) in type 1 diabetes carried out by the International Diabetic Nephropathy Study Group. The study enrolled 243 patients ages 10-40 years (16.8 +/- 6.0, mean +/- SD) with type 1 diabetes for 2-20 years (8.0 +/- 4.2) at centers in the United States (Minneapolis), Canada (Montreal), and France (Paris). At baseline, all patients were normotensive, none had reduced glomerular filtration rate (GFR), and all but eight were normoalbuminuric (NA). All patients had baseline renal biopsies. During the study, patients will have multiple measurements of blood pressure (BP), renal function, albumin excretion rate (AER), glycemia, and other variables, with repeat renal biopsies planned at 5 years after baseline. The 31.3% of the approached patients who agreed to participate were similar in age, diabetes duration, HbA(1c), AER, and sex to those refusing participation. Age, diabetes duration, HbA(1c), and AER were similar among the three centers, but systolic BP, GFR, renal plasma flow (RPF), and filtration fraction were lower in the Paris center. The 153 patients with hyperfiltration (GFR >130 ml x min(-1) x 1.73 m(-2)) had greater RPF than those with normal GFR. The eight microalbuminuric patients tended to have longer duration of diabetes but were otherwise similar to the NA patients. The role of these and other variables in determining the development rate of the early lesions of DN over the 5 years between biopsies is the central issue under study.