Heparin has been widely used in the prevention and treatment of various thromboembolic disorders for many years, and is considered to be a relatively safe drug. However, a small but significant number of patients receiving heparin develop thrombocytopenia and are at risk for associated complications. Recent studies suggest that the frequency of heparin-induced thrombocytopenia (HIT) is approximately three per cent with unfractionated heparin.