Blood volume was measured in 60 consecutive male patients with disabling angina pectoris undergoing preoperative investigation for coronary bypass surgery. Of these patients, 58 had a lower blood volume than predicted. There was a decrease in blood volume of 21% (1.3 litres) from the predicted normal value (P less than 0.001). Seventeen per cent of the patients were on diuretics, 30% were on digitalis, and 48% were on beta-blocking drugs at the time of the study. There was no correlation between this medical treatment and the deviation in blood volume. In patients without signs of myocardial insufficiency the decrease in blood volume was most prominent, while in those with an enlarged heart and a raised left ventricular preload (at rest) the deviation was less.