Markedly heterogeneous increases in hematocrit [Hct] readings were noted in a group of 9 patients [with renal disease] who previously had undergone hemodialysis, none of whom had polycystic disease. They were receiving continuous ambulatory peritoneal dialysis (CAPD) for at least 1 yr. There was no correlation between change in Hct reading and serum urea nitrogen, creatinine, total protein, or albumin levels. Respective contributions of blood transfusions and venipuncture were likewise negligible. The increase in Hct reading in several patients was gradual and still ongoing at 1 yr of CAPD. Other patients, 2 of whom had a decrease in transfusion requirement with CAPD, had either no increase in Hct reading or a small increase that was not sustained. A return to hemodialysis (or in 1 case, discontinuation of all dialysis with partial return of renal function) lowered Hct readings in patients who had initially responded to CAPD. The individual response in Hct reading to CAPD maintenance therapy is highly variable and is not predictable from clinically available measurements.