Acquired cystic kidney disease has been related to improvement of anemia in dialysis patients. It has been suggested that this could be due to erythropoietin production by the cysts. We studied 110 patients, 58 on hemodialysis and 52 on continuous ambulatory peritoneal dialysis, with an age of 48.6 +/- 14.78 years and a time on dialysis of 44.5 +/- 35.53 months. A renal echography was performed in every patient, evaluating presence and number of cysts. These findings were related to the blood levels of hemoglobin, ferritin, and erythropoietin as well as to the number of transfusions prescribed during the year of the study. The serum erythropoietin level was 18.23 +/- 12.14 U/l in hemodialysis patients, 15.04 +/- 12.35 in patients on continuous ambulatory peritoneal dialysis, and 12.4 +/- 4.7 U/l in the control group. Hemoglobin and erythropoietin were significantly higher in patients with polycystic kidney disease. Patients without cysts had the lowest levels of hemoglobin and erythropoietin, although no significant difference was found in those with multiple bilateral cysts or in those with 1-3 isolated cysts.