We report a case of primary, high-grade non-Hodgkin B-cell lymphoma in the breast of a young woman. The clinical and sonographic presentation was not of a mass but of an infiltrating anechoic process mimicking mastitis. Primary breast lymphoma is a rare entity, especially in young females. Early detection and treatment are crucial because the results of salvage treatment are generally poor. In previous imaging reports of breast lymphoma, it has always been considered as a mass, though the presence of markedly hypoechoic regions that look like fluid collections is a well known sonographic characteristic of lymphoma.