Serum erythropoietin levels were randomly collected and measured by a sensitive radioimmunoassay in a hemodialysis population. For analysis, the patients were divided into two groups: those with polycystic kidney disease and those with other kidney diseases. In 12 polycystic kidney disease patients, serum erythropoietin was 22.6 ± 2.4 mU/ml, hematocrit 29.7 ± 1.0%, and absolute reticulocyte count 17.0 ± 4.1 × 104/μl. In 24 other kidney disease patients, serum erythropoietin was 12.4 ± 0.7 mU/ml, hematocrit 21.2 ± 0.8%, and reticulocyte count 7.5 ± 1.5 × 104/μl. Serum erythropoietin was 18.5 ± 0.7 mU/ml in normal controls. Polycystic kidney disease patients manifested higher hematocrit, reticulocyte counts, and serum erythropoietin levels when compared to other kidney disease patients (p < 0.01). The data suggest (1) an inappropriately low serum erythropoietin level for the severity of anemia in uremic hemodialysis patients and (2) that greater availability of erythropoietin results in more effective erythropoiesis, even in the uremic e