Objective To determine if a diet complemented with calcium caseinate is better than a natural high protein diet for increasing serum albumin levels in patients on continuous ambulatory peritoneal dialysis (CAPD). Patients and Methods A 4-month clinical trial involving 100 patients older than 18 years was performed. Patients were randomized into two groups: group A, high protein diet (1.4 g natural protein/kg target weight/day and 35 kcal/kg target weight/day); and group B, calcium caseinate (0.7 g calcium caseinate plus 0.7 g natural protein diet/kg target weight/day and 35 kcal/kg target weight/day). Serum levels of albumin, total proteins (TP), BUN, creatinine, glucose, urea, sodium, and potassium, and hematocrit, leukocytes, erythrocytes, and hemoglobin were analyzed at baseline and every 30 days. Results The final mean albumin value was, for group A, 3.04 ± 0.39 g/dL, and for group B, 3.1±2 0.41 g/dL ( p < 0.05); TP for group A, 6.29 ± 0.47 g/dL, and for group B, 6.49 ± 0.51 g/dL ( p < 0.05); leukocytes for group A, 688 ± 8 1282/mm3, for group B, 7288 ± 1878/mm3 ( p = 0.05); BUN for group A, 47 ± 11 mg/dL, for group B, 50 ± 16 mg/dL ( p = 0.05). Regression analysis showed a treatment effect in serum albumin and TP levels from the third month in both groups. In group B, a constant elevation of serum albumin of 0.19 mg/dL and TP of 0.27 mg/dL was observed in every month of treatment with calcium caseinate. In the regression analysis of group A we observed a smaller increase in serum albumin, 0.06 mg/dL, and in TP, 0.11 mg/dL, in each month of treatment with the high protein diet. Both differences are significant ( p < 0.05). Conclusion Calcium caseinate used in CAPD patients suffering from malnutrition increases serum albumin levels.