A placebo-controlled, double-blind study with the stable prostacyclin analog Taprosten (CG4203, Grünenthal, Aachen, FRG) on the treatment of sudden hearing loss was conducted in 22 patients in the ENT Department of the University of Cologne. The hospital ethical committee gave permission for the trial. After informed consent had been obtained, the patients (13 male, 9 female, age 21-74 yrs.) were treated with Taprosten (25 ng/kg bodyweight/minute intravenously) or with 30 ml saline solution (NaCl 0.9%) for 6 hours during 5 days. Hearing thresholds were controlled by pure tone audiometry and speech discrimination test each day before and after treatment. All the data obtained (dB hearing loss) by pure tone audiometry was averaged for 7 frequencies. Differences were calculated between the hearing loss before and after treatment. Differences were considered significant if p less than or equal to 0.01. No significant difference could be seen in hearing recovery after Taprosten and placebo during the 5 days of treatment. Evaluation of the individual frequencies showed no prevalence of higher or lower frequencies. Although the impression was that hearing loss recovered better within the first days of treatment with Taprosten, no statistical proof could be found. In both groups the percentage of complete recovery did not differ (about 70%). The reason for the lack of significant statistical differences was the high standard deviation of the averaged hearing loss and the very high spontaneous recovery. Even if, in this placebo-controlled study, we could not prove the efficacy of a prostaglandin treatment for sudden hearing loss, the need for multicentred placebo-controlled studies with more patients is evident.