Mouse mammary tumor virus (MMTV) is the causative agent of breast tumors in mice. Recently, DNA sequences homologous or closely related to MMTV env gene have been specifically detected in breast cancer tissue from significant numbers of American, Australian, and Tunisian women, suggesting a viral etiology for at least a part of human breast cancer. However, the viral sequences have not been detected from any of breast cancer samples in several subsequent studies. Thus, whether MMTV-related retrovirus is a causative agent of human breast cancer remains controversial. To demonstrate if MMTV-related retrovirus is involved in Japanese cases of breast cancer, breast tissue specimens from 46 breast cancer patients and 3 patients with benign mammary tumors were investigated. Extensive analysis using PCR and Southern blot hybridization, however, could not detect the MMTV env gene-like sequence in any of the samples tested as well as in MCF7 cells that has previously been described as a positive control. Thus, MMTV itself or MMTV-related retrovirus is not associated with breast carcinogenesis in Japanese women, and it is unclear whether this conclusion is merely a reflection of regional differences in its epidemics.