In view of recent interest in the relationship of haematocrit and blood viscosity to hypertension and vascular disease, we have analysed retrospectively the relationship of haematocrit to blood pressure, vascular complications and other variables in 2,381 patients referred to the Glasgow Blood Pressure Clinic. Haematocrit correlated negatively with age and systolic pressure in men, and positively with age and systolic pressure in females. Positive correlations were found in both sexes between haematocrit and serum alanine aminotransferase (possibly due to mutual correlation with alcohol); and between haematocrit and overweight (Quetelet Index) but not obesity (Ponderal Index). Increased haematocrit was also associated with cigarette smoking; and with history of angina, myocardial infarction and intermittent claudication in females. No correlation was observed between haematocrit and history of stroke. These findings suggest that prospective studies of haematocrit in hypertensives may be of interest.