In order to determine the functional benefits of Cordyceps militaris in the immune system, we examined the immunomodulatory activities of Cordyceps militaris in an immunocompromised C57BL/6 mice model. Mice were injected intraperitoneally with an immunosuppressive drug, cyclophosphamide, and then administered orally with 3% hydroxypropylmethylcellulose or 30, 100, and 300 mg/kg of 50% ethanol extract of Cordyceps militaris (CM 30, CM 100, and CM 300, respectively) for 12 days. Mice treated with CM displayed significantly increased splenocyte proliferation and natural killer cell activity compared to immunosuppressed control mice (p less than 0.05). The spleen cells isolated from mice treated with CM also displayed increased production of Th1 cytokines, including IL-2, IL-12, IFN-γ and TNF-α, suggesting enhanced cellular immunity in response to CM. However, CM had no significant effect on the production of IL-4 and IL-10. These results indicate that Cordyceps militaris enhances immune function by promoting immune cell proliferation and Th1 cytokine production.