This study evaluated the nutritional status of 95 patients with chronic renal insufficiency who participated in the feasibility phase of the NIH funded Modification of Diet in Renal Disease (MDRD) Study. All patients were seen monthly by a physician and dietitian. After a baseline period, the patients were randomly assigned either to a mildly protein-restricted control diet or to one of three low protein, low phosphorus diets. Patients with clear evidence for protein-calorie malnutrition were excluded from the study. Patients were followed for an average of 12.4 months (range, 0 to 22 months). The glomerular filtration rate (GFR) at the commencement of the experimental diets ranged from 8 to 56 ml/min/1.73 m2. Assessment of the nutritional parameters indicated that none of the four diet treatment groups developed protein-calorie malnutrition. At the end of treatment with the experimental diets, most nutritional parameters were normal and few had worsened. Nonetheless, the following observations indicated that some patients had developed subtle evidence for protein-calorie malnutrition. There were positive correlations between the GFR and the serum transferrin and creatinine:height ratio. In men, arm muscle area and, at the onset of the experimental diets, the % standard body weight also correlated with the GFR. In women, GFR correlated with dietary energy intake. When patients were grouped according to their GFR level, those with the lowest GFR also tended to have lower energy intakes, serum transferrin levels and creatinine:height ratios. Patients with a GFR of 24 ml/min/1.73 m2 or lower tended to lose body mass during the study. In all groups of patients, the estimated actual energy intake was significantly lower than the prescribed intake. On the other hand, in the patients assigned to one of the three low protein, low phosphorus diets, nitrogen intake was above the prescribed level. The low energy intake of those patients with the lower GFR levels may contribute to their propensity to become malnourished.