This study was conducted to determine the incidence, severity, and causes of delayed vasospasm after clipping of unruptured paraclinoid aneurysms of the internal carotid artery (ICA). A retrospective analysis was made of 30 patients, who underwent clipping of unruptured paraclinoid aneurysms in our institution between 1991 and 1998. We compared angiograms before and after operation and classified them into two groups: vasospasm group and non-vasospasm group. Eleven variables were assessed as to their relationship to delayed vasospasm. There were 9 patients (30%) in the vasospasm group, of which 3 patients (10%) were clinically symptomatic. For all symptomatic patients, aggressive treatment, including triple-H therapy, was conducted with good outcome. The number of clips used (p<0.04) and temporary occlusion of the ICA (p<0.005) were statistically significant factors associated with the incidence of vasospasm. It is suggested that mechanical stimulation to the vascular wall of the ICA is responsible for causing spasm in addition to intraoperative bleeding around the dural ring.