Plasma viscosity was measured by the capillary method in 108 patients with ischemic heart disease. The highest value of plasma viscosity was found in 11 patients with severe unstable angina (1.66 ± 0.068), while in 18 patients with less severe unstable angina plasma viscosity was lower (1.61 ± 0.056; p < 0.025). In 43 patients with acute myocardial infarction plasma viscosity was 1.53 ± 0.10, significantly lower than in the two groups with unstable angina (p < 0.005). In 36 patients with stable angina plasma viscosity was 1.42 ± 0.089, similar to that found in 100 normal subjects. Plasma viscosity did not increase in 30 ischemic heart disease patients during exercise-induced myocardial ischemia. It is suggested that the elevated plasma viscosity in unstable angina demonstrated in this study compromises the oxygen delivery to the myocardium and coronary blood flow and therefore may possibly be a factor in the pathophysiology of this syndrome.