The rate of progression of early renal failure was evaluated in three groups of adult patients with renal disease of diverse etiology on dietary protein and phosphorus restriction (about 0.6 g/kg of protein, 700 mg of phosphorus) and in a control group of 22 patients with the same renal disease, retrospectively studied, on a free diet. Group 1 had 33 patients with chronic glomerulonephritis (CG), initial serum creatinine (Scr) of 1.4 to 4.3 mg/dl (mean, 2.20), followed for 5 to 94 months (mean, 44). Group 2 had 17 patients with polycystic kidney disease (PKD), Scr 1.3 to 4.7 mg/dl (mean, 2.40), followed for 8 to 81 months (mean, 42). Group 3 had 28 patients with primary chronic pyelonephritis (CP), Scr of 1.5 to 4.5 mg/dl (mean, 2.57), followed for 9 to 92 months (mean, 41). The control group had 22 patients (11 with CG, five with PKD, and six with CP), with Scr 1.7 to 4.1 mg/dl, followed for 6 to 72 months (mean, 24). In the regression analysis between reciprocal creatinine and time, the slopes were -0.0017, -0.0025, and -0.00016 dl/mg/month in the three patient groups on a protein-restricted diet, respectively. The difference between both groups 1 and 2 and group 3 was statistically significant (P less than 0.05). The slopes in patients on a free diet were significantly greater than those found in patients on a protein-restricted diet. The actuarial survival probability at 72 months, assuming as "renal death" a Scr of 10 mg/dl, was 45% in patients with CG, 44% in those with PKD, and 67% in those with CP on a protein-restricted diet.(ABSTRACT TRUNCATED AT 250 WORDS)