The aim of this retrospective study was to evaluate the effect of sealing of the round window membrane in patients with severe to profound unilateral sudden sensorineural hearing loss (SSNHL). 101 Patients with unilateral SSNHL were treated with tympanotomy and sealing of the round window membrane if hearing did not improve after conservative treatment. Preoperative and postoperative pure tone audiograms after removal of the ear packing were evaluated. A 4-PTA (pure tone audiometry) was used as reference value. The improvement of 4-PTA was analysed; in addition, recovery was evaluated using Siegel's criteria. Mean initial hearing threshold was 101.1 dB. Eighty-one patients had a hearing threshold of 80 dB or more. The average improvement at the time of ear packing was 21.7 dB and a further average recovery of 13.4 dB was recorded in the follow-up. Patients who underwent rapid tympanotomy within 5 days showed a significantly better hearing improvement than patients with delayed tympanotomy (26.9 vs. 14.0 dB, p < 0.02). Age was significantly correlated with the degree of hearing improvement. There was no significant difference of recovery between patients with detected lesions of the round window membrane and those without. Concomitant vertigo and tinnitus showed no significant effect on recovery. Tympanotomy and sealing of the round window membrane is effective in the treatment of severe to profound SSNHL. There is evidence that early surgery performed within 5 days is more effective than later surgery. The existence of a detectable lesion of the round window membrane has no significant influence on recovery.