The rupture of the round window membrane with consecutive perilymph fistula has gained in importance in the discussion of the pathogenesis of sudden hearing loss. In a retrospective study, the results of 23 tympanoscopies performed at the ENT Department of the University of Hamburg are reported. Two confirmed perilymph fistulas were seen, whereas in three cases they could not be excluded. After obliteration of the window niches with connective tissue an improvement of the hearing loss was seen in 9 cases. In comparison with our intraoperative findings, the number of perilymphatic fistulas mentioned in literature often seems very high. Nevertheless, tympanoscopy and obliteration of the window niches have their definite place in the therapeutic concept of sudden hearing loss, although the indication must be severely restricted.