Erectile dysfunction (ED) is a consequence of a number of medical conditions, with a high prevalence among men with type II diabetes. We investigated the effect of intracavernous injection of human umbilical cord blood derived endothelial progenitor cells (EPCs) on ED in a type II diabetic rat model. Erectile function was first assessed by cavernous nerve electrostimulation in Zucker Diabetic Fat (ZDF) rats aged 24 weeks to establish the baseline intracavernous pressure (ICP). At the age of 28 weeks, severe ED was confirmed. ZDF rats at age 29 weeks were randomised into 3 groups: treatment with 1million and 2million EPCs, and control, PBS. The injected animals were maintained with daily cyclosporine (10mg/kg) for 4 weeks, followed by the evaluation of erectile function and immunohistochemistry of penile tissue. The maximum (mean±SD) ICP observed for EPCs treated animals were significantly higher, 24.7±7.4 and 42.1±5.2 mmHg for 1million and 2million EPCs respectively in comparison to 4.4±1.6 mmHg in the age-matched PBS treated animals (p<0.001). EPCs treatment also improved the smooth muscle content of the cavernosum (p<0.001) and the expression of eNOS, RECA-1 and nNOS (p<0.05) compared to the PBS control. In conclusion, we showed for the first time that human cord blood derived EPCs significantly improved severe ED in ZDF rats in a dose-dependent manner through improvement of the nerve and endothelium function and also through preservation of smooth muscle cells in the cavernosum.