Haemorrheological variables were studied in 43 patients after acute myocardial infarction. Red cell deformability, by a filtration method, was significantly lower within 12 hours of infarction than subsequently. This drop was greater in the presence of haemodynamic complications. Blood viscosity, particularly when adjusted to a standard haematocrit, rose in the week after infarction, as did plasma viscosity and plasma fibrinogen. Haematocrit, however, fell over this period. These changes could increase myocardial ischaemia and lead to extension of the area of infarction.