The incidence of heparin-induced thrombocytopenia (HIT) types I and II was investigated retrospectively in 500 heparin-treated patients. In our department of internal medicine 500 patients were treated with unfractionated, middle- or low-molecular heparin (s.c. or i.v.) during 1995 as inpatients. Excluded were patients with other known causes of thrombocytopenia. There were 306 females (61%), mean age 76 years (17-98 years) and 194 males (39%), mean age 73 years (24-93 years). The incidence of HI type I was 4.4% and of HIT type II 0.6% with a positive heparin-induced-platelet-activation test (HIPA test). In addition, HIT type II was suspected in another 1.4% of cases with negative or missing HIPA test. Because of our results on the incidence of HIT type I and type II a close control of thrombocyte count during heparin therapy is necessary. In the case of HIT type II disease, heparin therapy must be stopped immediately.