Primary breast anaplastic large cell lymphoma (ALCL) is rare but is more commonly seen in patients with implants; fewer than 50 cases of breast implant-associated ALCL have been reported in the English language literature. Breast implant-associated ALCL is not a disease of the breast parenchyma, but instead is a disease of the fibrous capsule surrounding the implant. The patients usually present with an effusion around the implant and, rarely, with a solid mass. Morphologically, the neoplastic cells are large, epithelioid, and pleomorphic, with abundant cytoplasm, vesicular irregular nuclei, and frequent mitoses. Occasional ''hallmark'' cells may be present. The lesional cells typically show strong and diffuse immunoreactivity for CD30 and often express T-cell markers, cytotoxicassociated antigens, and epithelial membrane antigen. Almost all reported cases are negative for anaplastic lymphoma kinase. Molecular genetic analyses have demonstrated T-cell receptor gene rearrangements. The differential diagnosis essentially includes poorly differentiated carcinoma, other lymphomas, and chronic inflammation. Once a diagnosis of lymphoma is established, it is important to exclude systemic anaplastic lymphoma kinase-negative ALCL involving the breast, primary cutaneous ALCL, and other CD30+ lymphoproliferative disorders. The patients with effusion-associated ALCL often have an indolent course and excellent prognosis, responding well to excision of the fibrous capsule around the implant (capsulectomy) and implant removal. In contrast, patients who present with a distinct mass may have a more aggressive course and poor prognosis, requiring chemotherapy and/or radiation therapy.