Preoperative haemodynamic studies were performed in 60 out of 64 patients operated upon for occlusive disease of the subclavian artery. The pressure differences over the occlusive lesion were recorded in 53 patients. Blood flow was studied in 56 patients with the aid of electromagnetic flowmetry. The average mean pressure difference was 30 mmHg in 6 patients with occlusion of the brachiocephalic trunk and 20 mmHg in 20 patients with left subclavian artery occlusion. In 3 patients with right subclavian occlusion it was 17 mmHg. The highest individual mean pressure differences were found in patients with multiple occlusive lesions in extracranial cephalic arteries. Stenoses of the brachiocephalic trunk and the subclavian arteries in general caused a lower average mean pressure difference than the occlusions. Vertebral blood flow in cases of occlusion of the proximal part of the subclavian artery was usually reversed.