This study sought to investigate monocyte procoagulant activity and Mac-1 expression after successful percutaneous transluminal coronary angioplasty (PTCA) in acute myocardial infarction (AMI). An increased leukocyte count is an important risk factor for subsequent adverse cardiac events in AMI. Cellular procoagulant responses may contribute to the risk of thrombotic events after AMI. In 20 patients with AMI serial venous blood samples were obtained before, 4, 8 hours, and daily after direct PTCA. Twenty patients with elective PTCA served as a control group. We measured leukocyte procoagulant activity with a 1-stage clotting assay, Mac-1 expression of monocytes by flow cytometry, concentrations of tumor necrosis factor-alpha, interleukin (IL)-1beta, IL-6, and IL-8 using immunoassays. Forty-eight hours after PTCA in patients with AMI, an increase in systemic IL-6 and C-reactive concentrations was found (p = 0.001, p = 0.008) associated with an increase in monocyte Mac-1 expression by 49 +/- 18% (p = 0.04) and followed by an increase in monocyte procoagulant activity by 140 +/- 63% 72 hours after PTCA (p = 0.01). None of these changes were detectable in the control group. No changes in the concentrations of the cytokines IL-1beta, tumor necrosis factor-alpha, or IL-8 were found. The present study demonstrates an increase in procoagulant activity along with an increase in Mac-1 expression on circulating monocytes after successful PTCA in AMI associated with an increase in systemic IL-6. These cellular procoagulant responses may limit the clinical benefit from timely reperfusion.