Erdheim-Chester disease is a rare non-Langerhans cell histiocytosis of unknown etiology, the commonest sites of involvement being the long bones, skin, orbit, pituitary and retroperitoneum. Breast involvement is rare, with only four reported cases in the English literature. We present a case of a 78-year-old female presenting with bilateral clinically malignant breast masses, with mammographic and ultrasound findings suggestive of locally advanced bilateral breast cancer. Core biopsies from both breasts showed identical features, with a diffuse xanthomatous infiltrate with scattered Touton-type giant cells and a patchy lymphocytic infiltrate. The cells were CD68 positive, and negative for S100, CD1a and a broad panel of cytokeratins. The patient has a background history of cerebrovascular disease with carotid artery stenosis, and subsequently developed rapid restenosis after carotid endarterectomy. With the combined clinical history and classic histological findings in the breast, a diagnosis of Erdheim-Chester disease was made. This is the fifth case report of Erdheim-Chester disease involving the breast, and only the second case with breast lesions as the presenting symptom. Perivascular infiltration is also a rare but recognized presentation of Erdheim-Chester disease. Histiocytic proliferations including ECD can mimic breast carcinoma clinically, radiologically, and histologically, and should be considered in the differential diagnosis of breast mass lesions.