Five cases of primary non-Hodgkin's lymphomas of the breast (stage IE in the Ann Arbor staging system) are described. No clinical or radiological feature has permitted to make the diagnosis before surgery. Histologically, all lymphomas are diffuse and B type. Three cases are high grade lymphomas, one of lymphoblastic type. Two cases are of lymphoplasmacytoid type, one of them associated with an occult invasive ductal carcinoma within the same breast, the other changing in high grade lymphoma at the recurrence time. Three of them reveal histological aspects of lymphoepithelial lesions and appear as mucosa-associated lymphoid tissue lymphomas (MALT). The differential diagnosis with a carcinoma is now easily done by immunohistochemical techniques. The importance of a diagnosis before surgery by cytopunction and drill-biopsy has to be emphasized, to prevent an useless mastectomy.