The influence of different protein intake on renal function was studied in 16 Type 1 (insulin-dependent) diabetic patients, aged 15-23 years, with onset of diabetes before puberty and with a duration of diabetes between 5 and 20 years. The glomerular filtration rate, renal plasma flow, albumin excretion rate, and blood pressure were examined in a cross-over randomised order after 10 days on isocaloric diets with either 10% (i.e. 0.9 +/- 0.06 g.kg-1.day-1) or 20% (1.9 +/- 0.1 g.kg-1.day-1) of the calories as protein, the latter being equal to the recommended diet. Dietary compliance was evaluated using fractional phosphate excretion and overnight urea excretion. Glomerular filtration rate was lower after the low-protein diet compared to the usual protein diet (p less than 0.001). Patients with glomerular filtration rate above +2 SD of the normal mean on the usual protein diet (n = 6) exhibited the steepest fall in glomerular filtration rate with a mean decrease of 20 ml/min compared to 7 ml/min in those with initially normal glomerular filtration (p = 0.01). Filtration fraction tended to decrease on low protein diet, more so in initially hyperfiltering patients (p = 0.09). Renal plasma flow remained unchanged. In patients with elevated glomerular filtration rate on usual protein diet, albumin excretion rate and systolic, but not diastolic blood pressure, were decreased on low protein diet (p = 0.03 and p = 0.01, respectively) but not in initially normal-filtering patients. Mean blood glucose and serum fructosamine were unchanged on both diets.(ABSTRACT TRUNCATED AT 250 WORDS)