Objective: This study was performed to confirm the effect of transplantation of human umbilical cord blood mesenchymal stem cells (UCB-MSCs) on hearing restoration in a sensorineural hearing loss (SNHL) animal model. Material and methods: UCB was collected from pregnant women after obtaining consent, and mesenchymal stem cells (MSCs) were extracted. We established an SNHL model and transplanted UCB-MSCs through the brachial vein of the guinea pigs. The animals were divided into 4 groups: animals with normal hearing, animals with SNHL, animals with SNHL and injected with saline, and animals with SNHL and transplanted with UCB-MSCs. Hearing tests were conducted at 1, 3, and 5 weeks, and the results were compared by grading auditory brainstem response (ABR) recordings and distortion product otoacoustic emissions (DPOAEs) for each treatment. Lastly, cochlear pathological features were examined, and surface preparations and morphological changes in each animal model were compared using hematoxylin and eosin staining and light microscopy studies. Results: In SNHL group, decreased DPOAEs and increased ABR threshold were noted. Furthermore, in the SNHL group, ABR hearing thresholds were unconverted and were similar to those observed in deafness. The transplanted UCB-MSC group showed a significant improvement in hearing threshold (40 dB) compared to that in all the SNHL group (80-90 dB). Examination of the SNHL animals' cochlear morphological features demonstrated a noticeable lack of spiral ganglion cells and also showed degenerated outer hair cells. However, the transplanted UCB-MSCs showed an increase in spiral ganglion and hair cells. Conclusion: Intravenous transplantation of UCB-MSCs can enhance hearing thresholds, outer-hair cells and increase the number of spiral ganglion neurons (SGNs). (C) 2012 Elsevier Inc. All rights reserved.