The coexistence of cerebral aneurysm and pituitary adenoma has been described previously. Most of such cases harbor functional tumors and anterior circulation aneurysms, with very rare cases of posterior circulation aneurysms. In this report, we present a case of subarachnoidal hemorrhage due to rupture of an undetected basilar apex aneurysm during microscopic transsphenoidal surgery for a nonfunctional pituitary adenoma. Subarachnoidal hemorrhage following transsphenoidal surgery is a rare event. The concurrence of posterior circulation aneurysm and nonfunctional adenoma is uncommon too. Neuroimaging of cerebrovascular circulation before surgical treatment of pituitary adenoma, although controversial, would be helpful to evaluate vascular involvement and rule out any potential concurrent cerebrovascular diseases. Open or endovascular treatment of unruptured intracranial aneurysms carries relatively low morbidity and may be considered prior to other elective intracranial procedures like transsphenoidal surgery.