A 21-year-old female with a 5-year history of prolactinoma was referred to our hospital because of cerebrospinal fluid leakage and meningitis immediately following a third transsphenoidal operation for a rapidly growing tumor. Histological examination of the tumor removed at the second transsphenoidal operation found atypical epithelial and sarcomatous components. Administration of bromocriptine, radiotherapy, and chemotherapy were initiated. Emergent craniotomy was required for rapid regrowth of the tumor. Histological examination found predominant sarcomatous components. Tumor growth was difficult to control. The patient died of subarachnoid hemorrhage due to rupture of a pseudoaneurysm involving the C₁ portion of the right internal carotid artery. Pituitary adenoma rarely shows malignant transformation. In this case, prolactinoma underwent malignant change to sarcoma. Development of the pseudoaneurysm may have resulted from surgical manipulation, radiotherapy, or tumor invasion.