BACKGROUND Leukocyte-platelet aggregates appear to be a stable and sensitive marker of platelet activation as suggested by studies in coronary heart disease. We tested the hypothesis that leukocyte-platelet aggregates are increased after ischemic stroke and investigated the contribution of different leukocyte subtypes to such increase. METHODS We serially determined granulocyte-, lymphocyte- and monocyte-platelet aggregates, using flow cytometry at days 1, 2, 3, 5, 7, 10, and 90 in patients with ischemic stroke (n = 45) and in age- and sex-matched healthy control subjects (n = 30). RESULTS Granulocyte-platelet aggregates (granulocytes with > or =1 platelet/microl) were more common in patients than control subjects from day 1 through day 10 (p < 0.04, respectively), but not on day 90 after stroke. The percentage of granulocytes forming aggregates was increased on days 1-3 after stroke but not at other time points. Lymphocyte-platelet aggregates were not more common at any time point after stroke. Total numbers and percentages of monocytes forming platelet aggregates were significantly increased on day 2 (p = 0.003), but not at other time points after stroke. CONCLUSIONS The 3 leukocyte subtypes showed different kinetics regarding aggregate formation with platelets after ischemic stroke. Increase of monocyte-platelet aggregates is short-lived and may reflect an acute reaction to cerebral ischemia, whereas granulocyte-platelet aggregate formation persists into the subacute phase, suggesting that they are a particularly sensitive parameter reflecting both prothrombotic and inflammatory processes after stroke.