Although the protein leak of early diabetic nephropathy is said to be purely a glomerular lesion, there is still controversy as to the existence of a tubular component. We have, therefore, assessed the urine of insulin-dependent diabetics for tubular proteinuria as a feature of early diabetic nephropathy. The urine of 25 patients with increased albumin excretion rate was analyzed by sodium dodecyl polyacrylamide gel electrophoresis. One patient showed high molecular weight proteinuria, 2 showed low molecular weight proteinuria and 2 patients showed both low and high molecular weight proteinuria. The urine was also analyzed for 3 tubular proteins by single radial immunodiffusion. No patient showed elevated beta-2-microglobulin, but alpha-1-microglobulin (A1M) (corrected for creatinine excretion) was elevated in 3 out of 25 patients including 2 of the 4 patients with a low molecular weight pattern. One of the patients with raised A1M also had raised retinol-binding protein concentration. We conclude that, in early diabetic nephropathy, proteinuria can have a proximal tubular, as well as a glomerular, component.