Eighty-one cases with vestibular neuronitis were examined. The diagnostic criteria were a sudden onset of vertigo without previous symptoms, spontaneous nystagmus towards the healthy side, totally extinguished caloric responses with 44 degrees C and 30 degrees C water irrigation and no involvement of hearing associated with the onset of the disease. The series was divided into a prospective and a retrospective group. The prospective group A was examined at the acute stage, about 1 month and 1 year afterwards. The retrospective group B fulfilled the same criteria as group A and was examined 1-8 years after the acute stage. The results of the acute stage in group B were analysed from the case history reports, electronystagmo- and audiograms. The preceding and predisposing factors and symptoms were inquired. The examination scheme included the clinical otoneurological examination, the nystagmographic, audiological and clinical neurophysiological measurements and the serological and hematological specimens were collected at the acute stage of group A to examine the role of virus infections in the etiology of vestibular neuronitis. The liquor specimens of 16 cases available in group A were analysed. A recent respiratory infection was reported by 9 cases (27.3 percent) in group A and by 18 cases (37.5 percent) in group B. The serological evidence (increase of IgM-antibodies) was observed in 1 case against influenza A and in 1 case against parainfluenza 3 and the hematological examinations revealed clues of virus infection in 6 cases (18.2 percent) of group A. Cell counts and protein analyses of the liquor specimens were within normal limits. Cases with arterial hypertension under medical control were observed in 15.2 percent of group A and 14.6 percent in group B. These figures do not exceed the age- and sex-correlated prevalence of arterial hypertension in Finnish population. The clinical symptoms included an acute chiefly rotatory vertigo associated with nausea and vomiting without subjective involvement of hearing. The prominent symptoms lessened gradually during the first week and most of the patients were able to their earlier work after one month. The prognosis of the disease was good. The clinical otoneurological findings of the acute stage included spontaneous nystagmus with Frenzel's glasses and disturbances of the vestibulospinal tests. These abnormalities improved markedly during the follow-up period. The results of electronystagmography were characteristic of a pure peripheral vestibular disorder. Nystagmic beats were observed almost regularly in the pendular eye-tracking test at the acute stage examination.(ABSTRACT TRUNCATED AT 400 WORDS)