A Perilymph Leak into the middle ear through a ruptured round window membrane results in the symptoms of hearing loss, tinnitus and vertigo, either singly or in combination. The case histories of thirteen patients with such a fistula are described, these patients having in common a predisposing incident which had led to a rise of C.S.F. pressure. Symptomatology and the results of investigation are analysed and operative technique and results discussed. While it appears that vertigo uniformly responds very satisfactorily to operative treatment the improvement in hearing loss and tinnitus is more difficult to predict.