The progression of hearing loss and the factors influencing it are not well understood in patients with acoustic neuroma (AN). We investigated the relationships between the progression of hearing loss and the clinical findings in 42 patients with unilateral AN whose hearing could be examined for a period of 6 months or more. There was a tendency for hearing to deteriorate progressively in the patients with retrocochlear hearing loss, or with a small tumor. Based on these results, we conclude that an operation for hearing preservation should be performed as early as possible in these patients.