Two cases of transient cortical blindness during coronary and graft angiography are described. In both cases, the onset of blindness was immediately preceded by manipulation of a guidewire in the left subclavian artery during attempts to selectively engage the internal mammary artery. Vertebral angiography was performed in the first patient and the appearances were compatible with multiple emboli in the posterior circulation. This patient was anticoagulated with intravenous heparin. His vision recovered within 72 h, but a mild visual agnosia persisted and it was only after nearly 3 weeks that all neurological deficits had resolved. The second patient recovered fully within 15 min. We postulate that emboli occurring as a result of manipulation of the guidewire close to the origin of the left vertebral artery was the cause of the cortical blindness in both these patients, although vasospasm as a contributory factor cannot be excluded. With an increasing need for investigation of patients after coronary artery bypass grafting, angiographers should be aware that selective internal mammary graft angiography may carry a higher than normal risk of neurological complications. The use of a guidewire to manipulate the catheter in the subclavian artery may increase this risk.