A multicentre, prospective trial was organised to clarify the role of protein restriction in the progression of chronic renal insufficiency (CRI). 456 adult patients were assigned either a low-protein diet (0.6 g/kg body weight daily; n = 226) or a "normal" controlled-protein diet (1.0 g/kg daily; n = 230) and were stratified into three groups (A-C) with increasing baseline plasma creatinine concentrations. Each patient was followed up for 2 years or until an endpoint (a doubling of the baseline plasma creatinine or a need for dialysis) was reached. The difference between the diet groups in cumulative renal survival defined by these endpoints (27 low-protein, 42 controlled-protein) was of borderline significance (p less than 0.06). The difference in renal survival between the low-protein and controlled-protein diet groups was of borderline significance in group A (0 vs 4 endpoints), significant in group B (10 vs 21 endpoints; p less than 0.025), and not significant in group C. There were no differences among the diet groups or subgroups in mean plasma creatinine concentrations, creatinine clearance, the slope of the plasma creatinine reciprocal, or mean blood pressures. Compliance was good in the controlled-protein group but poor for the low-protein diet: the difference in protein intake between the groups was substantially less than that required by the protocol. However, there was no correlation between the progression of renal failure and protein catabolic rate. These findings offer little, if any, support to the hypothesis that protein restriction retards CRI progression: careful medical care and a "normal" controlled protein intake also allow very slow progression of CRI.