We have evaluated blood viscosity parameters in 20 men suffering from coronary heart disease (CHD) and in 15 control subjects. Whole bloodviscosity at a standardized haematocrit of 45% was significantly increased in the CHD-patients, both at low (p less than 0.001) and high (p less than0.05) shear rate. The increased whole blood viscosity in these patients was explained by an increased plasma viscosity (p less than 0.01), while the erythrocyte suspension viscosity values at a standardized haematocrit of 70%, reflecting erythrocyte deformability, were within the normal range. We have studied the effects of a daily supplementation of 1.5 g omega-3 polyunsaturated fatty acids on blood viscosity in the CHD patients. After 6 weeks of treatment whole blood viscosity, at low and high shear rate, and plasma viscosity were significantly improved (all p less than 0.05), although not normalized. There was no effect of the fish oil on erythrocyte deformability.