BACKGROUND: Sex steroids have immunomodulatory effects. No data exist on alterations of sex steroids during cardiopulmonary bypass (CPB). Cardiac surgery with CPB releases an immunological response with complement and cytokine activation. RESULTS: Plasma estradiol and progesterone levels before and immediately after CPB were measured in 11 patients. During CPB, mean estradiol levels decreased from 29 to 15 pg/ml and progesterone levels rose from 0.13 to 0.90 ng/ml. These changes were statistically significant. CONCLUSIONS: These are the first preliminary results evaluating plasma levels of sex steroids during CPB. Whether alterations in estradiol and progesterone levels influence complement and cytokine activation during bypass or vice versa is currently being investigated.