Giant aneurysm of the basilar artery presenting as a 3rd ventricular tumor is an unusual phenomenon. We are reporting a case in which a patient with a giant aneurysm of the basilar artery presented with symptoms of headaches and gait disturbance secondary to obstructive hydrocephalus. Although giant aneurysms presenting as mass lesions have been reported, the computed axial tomographic findings in our case were unique. Giant aneurysms of the basilar artery may be considered in the differential diagnosis of 3rd ventricular tumors.