Mouse Mammary Tumour Virus (MMTV) causes breast tumours in mice, and has been implicated in the aetiology of murine lymphomas. Several recent human studies have detected MMTV-env DNA sequences in 30-40% of tumours from breast cancer patients but in less than 4% of normal breast tissue. MMTV DNA has been detected in simultaneously diagnosed primary lymphoma and breast cancer tissue, suggesting MMTV may be involved in the aetiology of both breast cancer and lymphoma. We further hypothesize that if lymphoma and breast cancer are both associated with a common aetiologic virus then women with prior lymphoma may have an increased risk of breast cancer and vice versa. Despite the significant increase in the incidence of both lymphoma and breast cancer over the last few decades, their aetiologies are not well understood. Intriguing evidence to date suggests that the aetiology of a subgroup of human breast cancers and lymphomas may have viral involvement, and this warrants further investigation.