The mammary mouse tumor virus (MMTV) has been related to human breast cancer in previous studies, but these have yielded contradictory results. An MMTV env gene-like sequence was detectable in a relatively high proportion (38%) of human breast cancer tissues. The aim of this study was to determine the proportion of this 660 bp MMTV env gene-like sequence in a population of Austrian breast cancer patients. We performed PCR, repeat PCR, and nested PCR. We did not find any exogenous MMTV env gene sequences in the 50 DNA samples of human breast cancer tissue nor in 22 breast cancer cell lines including MCF-7, which has previously been described as a positive control.