To study the production of anti-platelet factor 4 (anti-PF4)/heparin complex antibodies of Ig (immunoglobulin) G/IgA/IgM using enzyme-linked immunosorbent assay (ELISA; heparin-induced thrombocytopenia [HIT] antibodies) in 79 patients undergoing cardiovascular surgery, we employed optical density (OD) as a marker of HIT-antibody production. The ODs were calculated from the differences in the ODs using ELISA. Patient were classified into 3 OD ranges: OD ≥ 1.0, OD ≥ 0.4 to <1.0, and OD < 0.4. The underlying disease, time course of the postoperative platelet count, <sc>d</sc>-dimer level, postoperative brain magnetic resonance imaging (MRI), use of cardiopulmonary bypass and postoperative thrombocytosis were not considered for the 3 OD classifications. None of the 6 patients with OD ≥1.0 and a positive functional assay was diagnosed with HIT due to the absence of HIT-derived thrombocytopenia. In conclusion, HIT-antibody production increased until day 7 after heparin cessation and reached a trace level on day 14. It was demonstrated that HIT-antibody production is in remission unless there is any evidence of a further increase during the second week postsurgery.