Regional blood flow, oxygen extraction ratio, and oxygen utilisation were measured in nine patients with breast carcinoma by means of the oxygen-15 steady-state inhalation technique and positron emission tomography. Regional blood volume was measured by means of tracer amounts of carbon-11-labelled carbon monoxide. The regional blood flow was consistently higher in non-necrotic tumour tissue than in surrounding normal breast tissue or contralateral normal breast. Oxygen utilisation was slightly higher in the tumour. The regional oxygen extraction ratio (ie, the fraction of oxygen extracted from the nutritional blood supply) was appreciably lower in the tumour than in normal breast tissue. These findings suggest that there is, at least on a macroscopic level, no supply-limited impairment of respiration in human breast cancer.