Doppler ultrasound was used to estimate the mean arterial flow in the arms of 50 patients with arm oedema following breast cancer treatment (oedema group). They were compared with 26 treated breast cancer patients with no arm swelling (control group). Flows on the treatment side were expressed as a percentage of the flow in the contralateral normal arm. Mean percentage blood flow was 68% higher in the swollen arms than the contralateral normal arms compared to a mean increase of 38% on the breast cancer treatment side in the control group. In both groups there was a significantly higher proportion of patients with increased (> 150%) rather than decreased (< 50%) flow on the treatment side. The ratio of increased to decreased flow was 27:4 in the oedema group and 6:0 in the control group. A neurological deficit on the treatment side was associated with an increased incidence of higher flow on the same side in the oedema group. These findings demonstrate that breast cancer treatment results in a significant increase in arterial flow in the arm on the treatment side, and that this increase is even higher in those patients with swelling. Increased blood flow is likely to contribute to arm swelling. One explanation for increased flow would be neurological deficit with loss of sympathetic vasoconstrictor control.