Glomerulotubular junction abnormalities, frequent in proteinuric patients with type 1 diabetes, may contribute to the progressive GFR loss in overt diabetic nephropathy. Glomerulotubular junction abnormalities were examined in patients who have type 1 diabetes with a wide range of albumin excretion rates (AER). Renal biopsies from five normoalbuminuric patients, five microalbuminuric patients, six proteinuric patients, and five control subjects were studied by light and electron microscopy. Light microscopy specimens were serially sectioned to find and classify glomerulotubular junctions. Glomerular structural parameters were estimated using stereologic methods. Glomerulotubular junction abnormalities were found in 2% of glomeruli from control and normoalbuminuric patients and in 4% of glomeruli from microalbuminuric patients. In contrast, 71% of glomeruli from proteinuric patients had glomerulotubular junction abnormalities, including five (8%) atubular glomeruli. Electron microscopy findings were typical of diabetic nephropathy. Piece-wise linear regression models with glomerular, glomerulotubular junction, and interstitial parameters as independent variables provided greater GFR (92%; P < 0.005) and AER (95%; P < 0.01) prediction than multiple regression models (81% for GFR and 72% for AER). Thus, glomerular adhesions and glomerulotubular junction abnormalities help to explain the progressive GFR loss that is associated with onset of proteinuria in type 1 diabetes. Moreover, nonlinear models provide better fit for structural-functional relationships in patients with type 1 diabetes.