Mouse mammary tumor virus (MMTV) is a well-known cause of mammary tumors in mice transmitted as endogenous proviruses or exogenously as infectious virions. The hypothesis that a retrovirus homologous to MMTV is involved in human breast cancers has resulted in renewed interest in the etiology of human breast cancer. Therefore, the detection of MMTV-like exogenous sequences in 30-40 % of invasive breast cancer has increased attention towards this hypothesis. To detect the prevalence of MMTV in Pakistani population, 666-bp-long MMTV envelop and 630-bp LTR sequences were amplified from breast cancer patient samples (tissue biopsies and peripheral blood) using mouse with mammary tumor as control. MMTV-like virus env and LTR DNA sequences were detected in 20 and 26 % of breast tumor samples, respectively, from the total of 80 breast cancer patients' blood and tissue samples. No significant association was observed between age, grade of disease, and lymph node involvement with the prevalence of MMTV-like sequences. Our data add to the growing number of studies implicating MMTV-like virus in human breast cancer, but still clear causal association of MMTV to breast cancer remains to be reputable.