Peripheral blood lymphocyte subsets were quantified by immunofluorescence in nine patients undergoing open heart surgery for coronary artery, valvular, and congenital heart disease. Compared with normal preoperative values, all patients developed an absolute lymphopenia, a reduction in T4 (helper) lymphocytes, and a statistically significant reversal of the T4/T8 ratio two hours after cardiopulmonary bypass (CPB). These changes could be caused by mechanical or immunogenic injury. A return to normal of the T4 subset and T4/T8 proportion occurred 24 hours after surgery. Whereas transient inactivation of immunoreactive lymphocyte clones may prevent unwanted immunization to blood products received during surgery, such temporary immune dysfunction could make certain patients liable to infectious sequelae. Viral-induced postperfusion syndromes, transmission of human T lymphotropic virus (HTLV) III by blood products, and reports of acquired immune deficiency syndrome after CPB foster a concern regarding postoperative infections under these circumstances.