Serum erythropoietin (sEpo) levels were serially measured with a radioimmunoassay in 14 patients undergoing autologous bone marrow transplantation (BMT), starting before the institution of the conditioning regimen up to day +45. An increase in sEpo levels was observed soon after starting the chemotherapy regimen, and before an evident fall in hemoglobin (Hb) levels took place. The peak in sEPo levels (221 +/- 181 mU/ml) was reached at day 0 in 9/14 patients, and was delayed up to day + 10 in the remaining five. There was a negative correlation between loge sEpo and Hb values (r = -0.730; p less than 0.01); the regression line of this correlation was comparable to the one obtained in a group of 15 iron-deficiency anemic subjects. Therefore, patients undergoing autologous BMT appear to be able to develop adequately increased sEpo levels in response to the severity of anemia. No correlation was found between sEpo and white blood cell or platelet count. On the other hand, sEpo value at day 0 was significantly related to the day of neutrophil recovery (r = -0.806; p less than 0.001): patients with the highest sEpo levels at day 0 showed significantly faster (p less than 0.001) neutrophil recovery.