The increasing use of peritoneal maintenance dialysis in end renal insufficiency calls for a better understanding of its advantages, and a comparative study of residual fluid as a partial substitute for serum may be helpful in this respect. A comparative biochemical study was carried out with 53 samples taking simultaneously from 16 patients in peritoneal dialysis. Statistical results showed linear correlation coefficient to be higher than 0.64 in BUN, phosphorus, creatinine, uric acid, calcium and potassium, which allows a statistically certain conclusion to be drawn to the serum levels of residual peritoneal fluid. The results of glucose, chloride, bilirubin, osmolality, LDH, bicarbonate, albumin, cholesterol, sodium, globulines and alkaline phosphatase studies, gave an insufficient linear correlation coefficient, which makes it impossible to offer the necessary guarantees for the use of residual fluid.