Erythrocytosis was found in 20 out of 127 patients who underwent renal transplantation (15.7%). Four were found to have elevated erythropoietin concentrations (139.7±22.0 um/ml) and 7 had normal levels (52.3+-84 um/ml (normal -60). The growth of erythroid CFUs was found to be normal in all patients. Theophylline treatment was given to 11 patients. The patients were divided into 3 groups according to their response: (1) patients with high erythropoietin levels whose hematocrit decreased from 59.6% to 46.3% within 2 weeks of treatment in parallel with a fall in erythropoietin; (2) patients with normal erythropoietin levels in whom theophylline therapy reduced the hematocrit from 58.7% to 47.2%; and (3) patients in whom theophylline had no effect on their erythrocytosis, We conclude that erythrocytosis after renal transplantation is a heterogenous condition and that several mechanisms are involved. In some patients, high erythropoietin levels or high sensitivity to erythropoietin are associated with the erythrocytosis. Theophylline therapy is beneficial in more than 50% of the cases.