Several studies have suggested that dietary protein quality may be an important determinant in the natural history of renal disease. We have therefore studied the effects of a predominantly vegetarian diet in eight patients with Type 1 diabetes mellitus and an albumin excretion rate (AER) in excess of 30 micrograms min-1. The AER was measured after an 8-week run-in period on the patient's usual diet, and again after 8 weeks of a predominantly vegetarian diet in which the proportion of vegetable protein was supplemented in order to minimize the reduction in total dietary protein intake. The median fractional albumin clearance fell during the study from an initial value of 188 x 10(-+) (range 58-810 x 10(-4)) at the end of the run-in period to 87 x 10(-4) (23-829 x 10(-4)) at the end of the period on low animal protein diet (difference 79 x 10(-4) (95% Cl 9-149 x 10(-4)), p less than 0.05). The AER then returned to values similar to those obtained at the beginning of the study after a further 8 weeks in those patients returning to their usual diet. No significant changes in blood glucose control or in arterial pressure were observed. A predominantly vegetarian diet may therefore have important beneficial effects on diabetic nephropathy without the need for a heavily restricted total protein intake.