The postoperative angiograms in 66 patients who underwent craniotomy for clipping of 78 cerebral aneurysms were reviewed. Indications for urgent postoperative angiography included neurological deficit or repeat subarachnoid hemorrhage. Routine postoperative angiograms were carried out in the remaining patients. Postoperative angiograms were reviewed to determine the incidence of unexpected findings such as unclipped aneurysms, residual aneurysms, and unforeseen major vessel occlusions. Logistic regression analysis was used to test if the following were factors that predicted an unexpected finding on postoperative angiography: aneurysm site or size; the intraoperative impression that residual aneurysm was left or a major vessel was occluded; intraoperative aneurysm rupture; opening or needle aspiration of the aneurysm after clipping; or development of a new neurological deficit after surgery. Kappa values were calculated to assess the agreement between some of these clinical factors and unexpected angiographic findings. Unexpected residual aneurysms were seen in three (4%) of the 78 occlusions. In addition, three aneurysms were completely unclipped (4%); these three patients were returned to the operating room and had their aneurysms successfully obliterated. There were nine unexpected major vessel occlusions (12%); six of these resulted in disabling stroke and two patients died. Of six major arteries considered to be occluded intraoperatively and shown to be occluded by postoperative angiography, two were associated with cerebral infarction. Logistic regression analysis showed that a new postoperative neurological deficit predicted an unforeseen vessel occlusion on postoperative angiography. Factors could not be identified that predicted unexpected residual aneurysm or unclipped aneurysm. The inability to predict accurately the presence of residual or unclipped aneurysm suggests that all patients should undergo postoperative angiography. Since a new postoperative neurological deficit is one factor predicting unexpected arterial occlusion, intraoperative angiography may be necessary to help reduce the incidence of stroke after aneurysm surgery. With study of more patients or of factors not examined in this series, it may be possible to select cases more accurately for intraoperative or postoperative angiography.