Plasma viscosity and erythrocyte aggregation, as the most important rheological factors in the microcirculation, and fibrinogen were measured in the blood of groups of patients in various stages of coronary-heart disease. Patients with unstable angina had viscosity and fibrinogen levels, even before any manifest infarction, that were higher than those of patients with stable angina. Plasma viscosity and hyperfibrinogenaemia (1.39 +/- 0.08 mPa.s in 48 patients and 394.4 +/- 82.7 mg/dl, respectively, in 33) were comparable to the values in patients with acute myocardial infarction (1.37 +/- 0.09 mPa.s [n = 45] and 390.2 +/- 126.9 mg/dl [n = 27], but significantly higher (P less than 0.02) than in those with stable angina (1.33 +/- 0.08 mPa.s [n = 78] and 295.3 +/- 68.6 mg/dl [n = 44], respectively). This abnormal viscosity in unstable angina plays a part in increasing myocardial ischaemia because oxygen delivery is already diminished and capillary flow slowed down. It thus contributes to progression of the angina and must be taken into account as an additional pathogenetic factor in the clinical instability.