In order to evaluate the pathophysiological relevance and clinical implications of leukocyte rheology in myocardial ischaemia we measured the percentage of aggregated leukocytes in 43 subjects with acute substernal pain before diagnosis. The percentage of aggregated leukocytes was significantly higher in 16 patients with subsequent diagnosis of myocardial infarction with respect to 11 with angina and 16 with non ischaemic chest pain (4.75 +/- 0.88, 3.43 +/- 0.65 and 1.52 +/- 0.32 respectively p < 0.01). The percentage of aggregated leukocytes was also evaluated in another group of 46 patients hospitalized for myocardial infarction. Among these, aggregated leukocytes were significantly higher in those with residual ischaemia, with respect to those without residual ischaemia (7.4 +/- 1.1 vs 3.5 +/- 0.6, p < 0.01). In conclusion, leukocyte aggregation is precociously increased after myocardial ischaemia. It may be a marker of residual ischaemia in patients with myocardial infarction.