Cardiopulmonary bypass (CPB) induces the release of platelet factor 4 (PF4) and patients are at risk of heparin-induced thrombocytopenia (HIT). This study was aimed to determine whether an abnormal evolution in platelet count (PC) after CPB is predictive of the development of HIT antibodies. Two abnormal PC patterns were defined: pattern P1, characterized by a decrease in PC following previous correction of thrombocytopenia occurring during CPB, and pattern P2, defined as a persistent low PC in the days following CPB. PC was evaluated for 10 d in 305 consecutive patients before and after CPB. Serotonin release assay (SRA) was carried out between days 8 and 10 to detect pathogenic heparin-dependent antibodies. Moreover, antibodies to heparin-PF4 (H-PF4) complexes were assayed by enzyme-linked immunosorbent assay. PC evolution after CPB was normal in 300 patients although antibodies to H-PF4 were frequently present (53.4%). Changes in PC were abnormal in five patients with pattern P1 (n = 4) or P2 (n = 1). As SRA was positive in four of the five cases, the positive predictive value of abnormal PC pattern for pathogenic HIT antibodies was 80%. Careful follow-up of PC after CPB makes it possible to predict with high specificity (99%) for those patients who develop pathogenic HIT antibodies.