Haemorrheological disturbances have already been described in ischaemic heart disease. However, it has not been established whether these changes are secondary to the ischaemia and/or myocardial infarction or whether they play a role in initiating or sustaining the haemodynamic abnormalities which cause infarction. We report our results observed in 14 patients aged 48 to 75 years admitted to the coronary care unit with a diagnosis of acute coronary insufficiency defined as typical persistent anginal pain resistant to glyceryl trinitrate associated with specific ECG changes (without pathological Q waves or increased serum CPK concentrations). Blood samples were obtained on admission for determination of: haematocrit, total blood viscosities at different levels of shear with the patients hematocrit and with corrected hematocrits, total blood filtrability, plasma viscosity and plasma albumin fraction. All patients received 800 mg lidocaine, 40 mg chlorezepate, adequate anticoagulant doses of heparin and a specific antianginal drug: amiodarone, nifedipine or diltiazem. Six patients had a favourable outcome and were discharged from the Coronary Care Unit without myocardial infarction (Group I); the remaining 8 patients (Group II) developed documented changes of myocardial infarction between the 12th and 4th day after admission (see the Table in the text). The haemorrheological parameters on admission of the two patients groups were compared. The abnormalities observed were significantly more severe in the group developing myocardial infarction. This suggests that these changes may play a major role in initiating conditions leading to myocardial necrosis. These observations confirm the results of other workers who have also shown a relationship between the severity of infarction and the incidence of haemodynamic complications and changes in blood viscosity and filtrability.(ABSTRACT TRUNCATED AT 250 WORDS)