Serum erythropoietin levels were measured by radioimmunoassay and compared to the severity of anemia in patients with end stage renal disease of different etiology, on chronic hemodialysis. It was demonstrated that the difference in severity of anemia in those patients is a consequence of a difference in erythropoietin production, rather than due to a difference in the level of erythropoiesis inhibitors. It was stressed that in patients with polycystic kidney disease the kidney tissue kept its endocrine function although it had no residual excretory renal function. The positive correlation between hematocrit values and erythropoietin levels indicates that in these patients erythropoietin synthesis is not regulated by general hypoxia. It is suggested that control of erythropoietin production in diseased kidney differs from normal physiological control.