There are two major approaches to basilar top aneurysm: one is subtemporal by Drake, another is pterional by Yasargil. In either approach, however, it is hazardous and difficult to approach basilar top aneurysm with megadolichobasilar anomaly because strong retraction of brain, nerve and vessel is required. A successful case of ruptured basilar top aneurysm with megadolichobasilar anomaly treated directly through the third ventricle is reported. A 62-year-old woman was admitted with consciousness disturbance. Plain CT scan on admission demonstrated massive and diffuse subarachnoid hemorrhage and a clot in the third ventricle. The left vertebral angiography demonstrated a 5 X 5 mm basilar top aneurysm with megadolichobasilar anomaly 24 mm distant from dorsum sellae. The aneurysm was considered to be inaccessible by conventional approaches because of its high position. Therefore, we have applied a new approach mentioned below. Operation was performed 2 days after the onset, using a bifrontal craniotomy. Dissecting interhemispheric fissure, we approached the third ventricle via lamina terminalis. Evacuating the clot in the third ventricle, the basilar top aneurysm was visualized. After dissecting the neck of aneurysm and perforators, clipping was performed. The postoperative course was uneventful. Surgical key points and prospective view in approaching via lamina terminalis and through the third ventricle for a high position basilar top aneurysm are discussed.