Acute myocardial infarction (AMI) in young people has recently received great attention. Its main characteristics include a typical risk factor profile and a good prognosis. We studied the haemorheological pattern (whole blood viscosity at high and low shear rates, plasma and serum viscosity, whole-blood filtration and erythrocyte aggregation) in 64 subjects, aged <46 years, with recent AMI. We observed marked alterations in blood, plasma and serum viscosity and whole-blood filtration. Subdividing the AMI subjects in accordance with the number of cardiovascular risk factors or the extent of coronary lesions, we did not observe any significative influence of these aspects on the haemorheological determinants, with the exception of low shear rate blood viscosity. The latter was in fact higher than in control subjects only in AMI subgroups with respectively more risk factors and more stenosed coronary vessels. Thirty-three AMI subjects were re-examined after three months and showed, on this occasion, a haemorheological pattern not significantly different from that of the first evaluation. These results demonstrate that in young subjects with AMI there is a hyperviscosity syndrome that persists during subsequent months, despite a good clinical course. Haemorheological impairment may unfavourably influence the long-term prognosis of AMI in young subjects and therefore, in our opinion, management should take into account the monitoring of the haemorheological pattern.