UNLABELLED: Anaplastic large cell lymphoma (ALCL) of the breast is a very rare nonepithelial neoplasm. In the literature, this tumor has sometimes been described in proximity of breast implants (60 implant-related ALCL reported). In 2010, a patient who had undergone a right mastectomy and tissue expander/implant reconstruction for a "ductal" carcinoma 10 years before was referred to our unit for evaluation. On examination, an enlarged reconstructed right breast was found. The reconstructed breast did not show tenderness or signs of infection, ulceration, or breakdown. Mammograms and ultrasound scan did not suggest the presence of recurrent cancer, infection, deflation of the implant, or severe capsule contracture. The patient underwent mammary implant replacement. About 3 weeks after surgery, the patient came back to our unit for a new mild enlargement of the operated breast and the implant was removed. Three months later, the patient returned with a skin lesion in the right parasternal region. A radical excisional biopsy was performed under local anesthesia and the diagnosis of ALK-1-negative ALCL was finally made. The clinical and histological diagnosis of this disease is difficult as it can often be mistaken for a simple seroma (breast enlargement), an infection, or an unspecific reaction to silicone (redness and/or tension of the skin, itching, and fever). We strongly suggest considering ALCL in any patient with a spontaneous breast seroma lasting more than 6 months after mammary prosthesis implantation. The suspicion of ALCL must be suggested to the pathologist immediately. 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 www.springer.com/00266 .