The human prostate cancer cell lines, PC-3 (androgen-insensitive) and LNCaP 104-R (androgen-repressed) were inoculated subcutaneously into nude mice to produce prostate tumors. Intraperitoneal injection of green tea (-)epigallocatechin-3-gallate but not structurally related catechins, such as (-)epicatechin-3-gallate, inhibited the growth and rapidly reduced the size of human prostate tumors in nude mice. (-)Epigallocatechin-3-gallate also rapidly inhibited the growth of tumor growth formed by the human mammary cancer cell line MCF-7 in nude mice. It is possible that there is a relationship between the high consumption of green tea and the low incidence of prostate and breast cancers in some Asian countries.