The relationship between arterial blood pressure and red blood cell variables was investigated in 1013 unselected persons with a blood pressure range of 90 to 225 mm Hg systolic and 50 to 145 mm Hg diastolic. Statistically significant correlations were demonstrated between mean (as well as systolic and diastolic) arterial blood pressure and red blood cell count (r = 0.27; P less than .0001), hematocrit (r = 0.28; P less than .0001), and hemoglobin concentration (r = 0.29; P less than .0001). Average arterial blood pressure was higher in men than in women (133 +/- 16/83 +/- 10 v 124 +/- 16/79 +/- 9 mm Hg, P less than .0001) and this was associated with higher values for erythrocyte count, hemoglobin concentration, and hematocrit in men as compared to women. The significant correlation of blood pressure and hematocrit, which represents one important determinant of blood viscosity, points to a role for rheological factors in the long-term control of blood pressure. Moreover, it might be speculated that the sex difference in blood pressure as observed in the present study may be due, at least in part, to stimulated erythropoiesis in men as compared to women.