A case of primary adult T-cell lymphoma (ATL) of the breast is described. A 69-year-old woman presented with a painless, rapidly growing lump in her left breast. Staging procedures demonstrated no sign of generalized disease. Following a Patey’s mastectomy, 10 courses of adjuvant chemotherapy (CHOP) were successfully administered. The light microscopic, immunohistochemical and molecular genetic analyses of the surgical specimen revealed a primary ATL. Seropositive mothers who breast-feed their children may facilitate the accumulation of T cells carrying HTLV-I in their breasts and therby increase their risks of developing breast ATL.