['The outcome of cardiac surgical patients with clinically diagnosed heparin-induced thrombocytopenia (HIT) was studied.', 'Retrospective, observational study.', 'University hospital.', 'All cardiac surgical patients with diagnosed HIT after cardiac surgery between January 2002 and December 2004, and concurrently, consecutive patients without HIT.', 'None Measurements and Main Results: 3465 patients were treated postoperatively in the cardiac surgical intensive care unit during the study period. Clinical suspicion of HIT arose when platelet count markedly fell several days after surgery and HIT was proven by a positive enzyme immunoassay in 20 patients. Thrombocytopenia (35.5 [22] x 10(9)/L, median [interquartile range]) developed within 7 (6) days. HIT patients received significantly more platelet transfusions perioperatively than controls (p < 0.001). Thromboembolic complications occurred in 70% of HIT patients, but in none of non-HIT patients (14/20 v 0/20, p = 0.001). Intensive care unit stay was longer in HIT patients than in controls (16.5 [11.0] v 1.0 [3.0] days, p < 0.001). Nine HIT patients died (45%), while all control patients survived. Mortality was related to thrombotic complications in seven HIT patients (35%).'] Is heparin-induced thrombocytopenia following cardiac surgery associated with poor outcome?