Intra-operative B.A.E.P. monitoring have been performed in two series of patients who underwent Microsurgical Vascular Decompression (M.V.D.) of the Vth and VIIth cranial nerves in the posterior fossa. The first series consisted of 17 patients among the 400 operated on for Trigeminal Neuralgia (T.N.), the second one also of 17 patients among the 25 treated for Hemifacial spasm (H.S.), over the last four years. The equal number of recorded patients in the two groups is totally fortuitous. The aim of the work was to correlate the electrophysiological changes: 1) to the surgical manoeuvres potentially responsible for B.A.E.P. alterations, in order to modify the M.V.D. technique, so as to minimize the risks of hearing loss, 2) to the eventual post-operative auditory disturbances, in order to define prognostic criteria from B.A.E.P. monitoring. I. Intra-operative B.A.E.P. modifications were graded into 4 categories, according to their degree of severity (table II): minimal changes (category 1), increase in latency returning to normal (2) or without normalization (2a) before end of surgery, abolition or partial decrease of responses returning to normal (3) or without complete normalization (3a) before end of surgery, total B.A.E.P. loss lasting through the entire procedure (4). Patients operated on for H.S. had more often significant B.A.E.P. changes than those with T.N., respectively 10 and 6 for categories 3a and 4 together. Auditory function was at risk mainly during cerebellar retraction (especially if cerebello-pontine angle was approached laterally) and during vascular manipulation of the labyrinthine artery (which can generat vasospasm).(ABSTRACT TRUNCATED AT 250 WORDS)