Sézary syndrome (SS) is a rare form of mature cutaneous T-cell lymphoma characterised by generalised erythroderma, lymphadenopathy and typical SS cells with cerebriform or convoluted nuclei in the peripheral blood. In patients with advanced disease, internal organs may be involved. Here we describe a 32-year-old woman, who had given birth to her first child 12 months before referral to our haematology unit with severe pruritus, multiple patches of erythroderma of four months duration and palpable inguinal and axillary lymphadenopathy. There was mild leucocytosis: WBC 11·7 × 109/l with 5·62 × 109/l lymphocytes. Characteristic Sézary cells (>1·0 × 109/l) were seen in the peripheral blood film. Skin biopsy showed a typical histopathological picture of SS. An 18F-fluorodeoxyglucose positron-emission tomography/computed tomography (FDG-PET/CT) scan, performed for staging, showed strong uptake in the axillary and inguinal lymph nodes and unexpectedly also in the left breast (right). A biopsy revealed islands of normal glandular breast tissue, surrounded by small slightly irregular CD3+/CD4+/CD8− T cells compatible with Sézary syndrome (left). T-cell receptor gene rearrangement was positive showing that the process was monoclonal. Tumourous breast involvement in SS is extremely uncommon.