A retrospective case record study of 20 patients in Oslo operated on for chronic otitis media with labyrinthine fistula between 1986 and 1999 was performed in order to estimate the incidence of, and identify predictors for, labyrinthine fistulas. The incidence of fistula was 0.3 per 100 000, with a median age at diagnosis of 37 years. The median duration of chronic otitis media prior to labyrinthine fistula detection was significantly correlated with age at surgery. Subjective hearing loss (90%), otorrhoea (65%) and dizziness (50%) were presenting symptoms. Modified canal-wall-down mastoidectomy was performed in all patients. Preoperative hearing levels could not predict postoperative hearing outcome. Positive signs of fistula were found in only 4 patients (20%). Correspondingly, computerized tomography (CT) diagnosed the fistula in 11 patients (55%). The seven patients presenting without dizziness and with a negative CT scan and fistula test were characterized by lower age, absence of previous middle ear surgery, lower preoperative pure-tone thresholds for bone conduction and better hearing outcome after surgery. In conclusion, the identification of a younger group of patients presenting with fewer symptoms indicates that fistulas should be suspected in all patients undergoing surgery for chronic middle ear and mastoid disease.