We have studied serum erythropoietin (EPO) levels during 6 years after kidney transplantation in 16 patients. There was a serum EPO peak around 50 mU/ml after 5 weeks. After 3 months the serum EPO level stabilized at around 30 mU/ml. Patients with good transplant function had significantly higher serum EPO levels and normalized their hemoglobin (Hb) after a mean of 3 months. If transplant function was good, Hb was normalized even if the serum EPO was only slightly elevated. Patients with poor transplant function had lower serum EPO and Hb levels. We concluded that a good transplant function is the key to a normal erythropoiesis and that small amounts of EPO are needed to improve Hb.