The Cimicifuga racemosa extract BNO 1055 (CR) inhibits proliferation of human prostate cancer-derived LNCaP cells. Therefore, we tested the capability of this extract to inhibit formation and/or proliferation of tumors induced by subcutaneous ( s. c.) inoculation of LNCaP cells in immunodeficient nu/nu mice. After inoculation of 1 million cells, 12 of 18 animals developed solid tumors while tumor development was seen in only 5 of 18 CR-treated animals and the size of the latter at termination of the experiments 10 weeks after inoculation was significantly smaller than in the control animals. Upon histological inspection, the amount of tumor tissue in the control animals was significantly larger than in the CR-treated animals while in the latter connective tissue was predominant. The CR treatment did not affect serum testosterone levels significantly regardless of whether the animals developed tumors or not. It is concluded that compounds in CR inhibit tumor development, proliferation and dignity of the tumors following s. c. inoculation of LNCaP cells. Hence, the CR extract may prove to be efficient in preventing and treatment of prostate cancer.