UNLABELLED: Breast implant-associated anaplastic large cell lymphoma (ALCL) is a rare new clinical entity. The incidence is 0.3 % per 100,000 women per year. Patients present with non-specific implant-related complications resulting in delayed diagnosis. We present such a case to raise awareness and discuss management. A 48-year-old female presented with a 3-month history of left breast pain and swelling. She had undergone multiple bilateral augmentations 8 years previously. Triple assessment revealed a seroma, and a magnetic resonance imaging scan excluded implant rupture. Cytology showed a typical cells with mitotic activity which lead to removal of implants and a left capsulectomy. Final histology revealed an anaplastic lymphoma kinase (ALK) negative ALCL confined to the capsule. A computerised tomography scan and bone marrow biopsy excluded systemic disease, but due to later identified B symptoms, she received CHOP chemotherapy under the care of the haematologists. ALK-negative ALCL is associated with breast implants, and any persistent late onset seroma or breast symptoms should raise the suspicion of ALK-negative ALCL as a differential diagnosis. The recommended treatment is surgical removal of the implant including a full capsulectomy, highlighting the suspicion of ALCL to the pathologist. Exclusion of systematic disease is also recommended in all patients, and the need for adjuvant therapy should be addressed on an individual case basis. For disease confined to the capsule, adjuvant chemoradiotherapy is not needed. LEVEL OF EVIDENCE V: This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors http://www.springer.com/00266 .