Low-protein, low-phosphorus diets (LPD) are prescribed to patients with chronic renal failure (CRF) to slow down the rate of progression of CRF and to control uraemic symptoms. A satisfactory adherence of patients to the prescribed diet is needed to meet these two goals. We studied the compliance of CRF patients to a LPD providing daily (per kg body weight) 0.3 g protein, 3-5 mg phosphorus, 35 kcal, and supplemented with essential amino-acids and keto-analogues. Forty CRF patients were studied for 23.3 +/- 10.8 months (range 12-45). Compliance to LPD was evaluated by dietary inquiry and protein intake estimated from urinary urea excretion. According to compliance to LPD, patients were retrospectively assigned to the compliant (n = 27) or the non-compliant (n = 13) group. GFR measured by the urinary clearance of [51Cr]-EDTA was identical in the two groups at the start of the study: compliant patients 15.7 +/- 5.3 ml/mn, non-compliant patients 15.4 +/- 5.9 ml/mn. The decrease of GFR was -0.08 +/- 0.22 ml/min per month in compliant patients versus -0.31 +/- 0.37 ml/min per month in non-compliant patients (P < 0.02). These results were not demonstrated if the progression of CRF was assessed by the linear regressions over time of creatinine clearance or the reciprocal of creatinine. Serum bicarbonate, serum phosphorus and PTH levels were corrected by LPD in compliant patients (P < 0.005 for all parameters) but not in non-compliant patients. These results suggest that evaluation of compliance is necessary to assess the response of CRF patients to LPD, whether the aim is to slow the progression of CRF or to control its metabolic consequences. A beneficial effect of compliance to LPD was demonstrated upon these two goals.