Heparin-induced thrombocytopenia (HIT) is occurring in a relatively low frequency but is sometimes problematic clinically. The aims of this study were to know the epidemiology of HIT in Korean population and to gain some insight of the physicians’ awareness of this syndrome. We obtained the records of all patients treated with unfractionated heparin (UFH) for their acute coronary syndrome at the Gachon Medical School Gil Heart Center during 2002–2003. Eligible patients were required to have checked complete blood counts at least twice in their hospital courses and to have normal baseline platelet count. Patients who had unexplained (absence of all common causes) thrombocytopenia were considered to have clinical HIT. We also reviewed the medical record of all patients with thrombocytopenia to determine whether the treating-physicians have thought and/or ordered any laboratory evaluation for immune-mediated HIT. Among 874 consecutive patients treated with UFH, 272 (31.1%) had never followed their platelet count after admission and 58 (6.6%) had baseline thrombocytopenia. Thus, 554 patients had protocol-compliant data and were included in the study. Median duration of UFH administration was 6 days (range 1 to 41) and 62 (11.1%) patients developed thrombocytopenia. Among the 442 patients who received UFH for > 96 hours, 51 (11.5%) developed thrombocytopenia. In these patients with prolonged UFH administration, 16 had a hematology consult, 30 had suspected to have clinical HIT, and only one had a diagnosis of HIT in their medical records. In 21 patients, possible other causes for thrombocytopenia were evident. Even though this is a tertiary academic hospital, serologic tests for HIT was never ordered. Keeping in mind the limitations of retrospective analyses, the present study shows that thrombocytopenia is common in patients treated with UFH and that physician’s awareness of this syndrome is limited.