Twenty-eight patients with locally advanced breast cancer received four doses of regional chemotherapy via angiographically placed percutaneous catheters into the internal mammary artery (IMA) and lateral thoracic artery. Patent blue dye was injected to outline the relative contribution to perfusion of each of these vessels. The IMA was found to perfuse 67 (range 20–95) per cent of the breast and the lateral thoracic artery 15 (range 0–35) per cent. In 33 per cent of patients the lateral thoracic artery did not contribute to breast perfusion and a large area of the lateral aspect of the breast was perfused from a further branch of the subclavian or axillary artery. The blood supply to the breast is extremely variable and must be determined in each patient before delivering regional chemotherapy.