Mesenchymal stem cells (MSCs) are candidates for cell therapy of kidney diseases. However, the application of MSC derived from human umbilical cord (UC-MSC) in treating acute renal failure (ARF) has not been reported. UC-MSCs, 10(6), were transplantated via the left carotid artery into ARF rats which was established by clamping bilateral pedicles for 60 min and reperfusing. Serum creatinine and urea nitrogen decreased 4.8 times and 3.6 times as well as caspase-3 and IL-1beta decreased 5.8 times and 9 times compared to control groups, respectively. The percent of proliferative cell nuclear antigen (PCNA)-positive cells (53% +/- 7.5%) was higher than that in the control groups (17% +/- 4.5%). In addition, the transplanted UC-MSCs could reside in local injury sites, leading to the relief of hyperemia and inflammation, but no obvious transdifferentiation into renal-like cells. The results lay the foundation for further study on the potential application of UC-MSC in human disease.