A total of 55 cases with unilateral acoustic neurinoma which were operated on by the lateral suboccipital approach was studied to elucidate factors which influence postoperative hearing acuity. We analyzed several factors: preoperative hearing level, tumour size, tumour consistency (cystic or solid), and anatomical location of the cochlear nerve. The size of the tumours ranged from 1.2 to 5.8 cm in diameter. Thirty of 55 cases (55%) preoperatively had remaining cochlear function. The smaller the size of tumour, the higher was the preoperative hearing level excepting those tumours with a diameter of 5 cm or greater, which had relatively good hearing and often contained large cysts. As to the consistency of the tumours, 41 were solid and 14 were cystic, where 19 (46%) and 11 (79%) cases had had preoperative hearing, respectively. Anatomical continuity of the cochlear nerve was maintained at surgery in 15 of 30 cases with preoperatively remaining hearing; cochlear function was preserved after surgery in 9 of the 15 cases. It was located counter-clockwise (caudally) to the facial nerve at an angle of 50 degrees on average when they were projected on the right side. The distance or interrelation between the two nerves had no bearing on postoperative hearing preservation. Postoperatively, hearing acuity was improved in 6 cases (20%) with a mean value of 5.6 dB, unchanged in 3 (10%), and deteriorated in 21 (70%) among the 30 cases with remaining preoperative-hearing. When the tumour was less than 2 cm or cystic, better hearing preservation was expected.(ABSTRACT TRUNCATED AT 250 WORDS)