To examine the relationships between the normal dietary intakes of protein and phosphate, blood pressure, and the progression of diabetic nephropathy, we prospectively studied 20 Type 1 diabetic subjects of mean age 43 +/- 10 years (SD) with early nephropathy (mean serum creatinine 115 +/- 43 mumol l-1) over 1 year. Three-monthly measurements of blood pressure, glycaemic control, and normal dietary intake (3-day weighed food records) and 6-monthly measurements of glomerular filtration rate (using a single injection of chromium 51-EDTA) were made. GFR changed at a median rate of -0.89 ml min-1 1.73 m-2 month-1 (range +0.85 to -2.55 ml min-1 1.73 m-2 month-1). Mean dietary protein intake (1.22 g kg-1; range 0.78 to 1.55 g kg-1) and phosphate intake (21.5 mg kg-1; range 14.1 to 30.4 mg kg-1) were not significantly related to the rate of change in GFR. Only mean systolic blood pressure was significantly related to change in GFR, and accounted for 45% of the variability in GFR decline in the 18 subjects who completed the study (r = 0.67; R2 = 0.449; F1,16 = 13.2; p < 0.005; 95% confidence interval for r: 0.336-0.867). A mean systolic blood pressure of 140 mmHg or below was associated with no significant decline in GFR over a median period of 13 months.(ABSTRACT TRUNCATED AT 250 WORDS)