Auditory brain stem responses (ABR) of 61 patients with tumors of the VIIIth nerve or cerebellopontine angle were analyzed for latency of the I-III, III-V, and I-V interwave intervals. Of the 61 patients, 16 yielded ABR records with repeatable waves I, III, and V so that interwave intervals could be measured. Results indicate that not only is the I-III interval often abnormal, but in almost 50% of the cases the III-V interval is also extended. Additional analysis included comparing a group of subjects with cochlear lesions with the 16 patients with VIIIth nerve tumors. These findings indicated a higher degree of specificity than sensitivity for the ABR interwave measurement. These findings, as well as additional analyses, are discussed.