The effect of a low protein diet (LPD) on the progression of chronic renal insufficiency (CRI) was investigated by reviewing the published studies. Only the trials of Rosman, Ihle and Locatelli fulfilled the main methodological criteria of being randomized, prospective and controlled. They involved 811 patients (671 evaluated: 338 on an LPD, 333 as controls) and had a mean follow-up of 29 months (range 18-48) for an estimated total of about 17,335 patient-months. The only trial whose results showed that LPD had a positive effect on the progression of chronic renal failure (CRF) was Ihle's study with the lowest weight (6.7%) and which involved the most severe CRF; effects limited to the patients with more advanced CRF were found in Roman's study, with an intermediate weight (41.8%); and little effect, if any, in Locatelli's trial accounting for 51.5% of patient-months, with less severe CRI. In conclusion, analysis of published randomized, prospective and controlled trials offers little or no evidence for the hypothesis that an LPD has a greater clinically significant effect on early CRI progression than a controlled protein diet, although a very low protein diet seems to postpone the need for dialysis.