We studied five patients who developed a decrease in platelet count of more than 30000 x 10(6)/l within 24 h of heparin treatment and direct percutaneous transluminal coronary angioplasty and/or intracoronary thrombolytic therapy among 38 consecutive patients with acute myocardial infarction (AMI). Anti-platelet factor 4 (PF4)-heparin complex antibodies were detected in the sera of these five patients, although they had never previously been exposed to heparin. These patients might have had specific antibodies before heparin treatment. PF4 might be released from activated platelets and bind to endogenous heparin-like molecules, and antibodies with cross-reactivity to the PF4-heparin complex may have been generated by the endogenous complex before the first heparin treatment. We conclude that it is worthwhile to check the platelet count and screen for anti-PF4-heparin complex antibody in advance to prevent thrombotic complications due to heparin treatment in patients with AMI.