In a study of the effect of a low-protein diet on the progression of renal disease 19 insulin-dependent diabetic patients with persistent clinical proteinuria were observed for 12-39 (mean 29) months while they were on a normal-protein diet (1.13 [0.06] g/kg per day), then for 12-49 (mean 33) months on a low-protein diet (0.67 [0.03] g/kg per day). The low-protein diet had no adverse effect on nutrition or glycosylated haemoglobin concentration. Mean supine blood pressure (BP) fell slightly on the low-protein diet and was probably due to the start or modification of antihypertensive medication in 9 patients. The mean rate of decline in glomerular filtration rate fell from 0.61 (SEM 0.14) ml/min per month with the normal-protein diet to 0.14 (0.08) with the low-protein diet, and this effect remained highly significant after adjustment for blood pressure, energy intake, and glycosylated haemoglobin. The rise in the fractional clearance of albumin during a normal-protein diet stopped with the low-protein diet, and there was a significant fall in albumin excretion from 467 (95% CI 234-895) micrograms/24 h on the normal-protein to 340 (138-719) on the low-protein diet. Thus, a low-protein diet, with its reduction in protein and possibly other dietary components such as phosphate or fat, seems to retard the rate of decline of glomerular filtration rate in diabetic nephropathy independently of blood pressure changes and glycaemic control.