Objective: To study the effect and mechanism of LHRH analogue nafarelin on the growth of BCaP-37 human breast cancer cell xenografts in nude mice. Methods: Nafarelin treatment began at next day or 2 weeks after tumor inoculation and each group had 3 different doses by s.c. per day for 4 weeks. Tamoxifen and another LHRH analogue D-Trp6 LHRH were used as positive control. The changes of xenografted tumor induced by nafarelin treatment were investigated with respect to its histology, cell DNA index (DI), FCM, serum level of E2 as well as the mass of tumor, uterus and bilateral ovaries. Results: (1) Chronic administration of nafarelin at doses of 500 or 1 000 μg/kg resulted in a significant inhibition of tumor growth and a decrease in DI and serum level of E2, apoptosis of tumor cells in nafarelin treated groups was also observed. (2) Tamoxifen had no inhibitory effect on the growth of xenografted tumor at 30 mg/kg. D-Trp6 LHRH had a potency comparable to nafarelin in suppressing the growth of BCaP-37 xenografted tumor. Conclusion: Chronic administration of nafarelin could inhibit the growth of xenografted BCaP-37 breast cancer significantly.