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No studies have reported breast cancer cases developed in the chronic expanding hematoma (CEH). Case presentation: A 47-year-old woman was referred to our hospital for the treatment of a large breast mass. Ultrasound showed that the tumor had an intra-cystic tumor pattern. Magnetic resonance imaging (MRI) of the mass component showed a hypo intense pattern on T1-weighted images, a mosaic pattern on T2 weighted images, and a faint enhancement on time-signal intensity images. Core needle biopsy pathologically showed connective tissue, hematoma, and hemosiderin laden macrophages neither with any mammary gland components nor with malignant cells. These image findings and the presence of hemosiderin laden macrophages led us to the diagnosis of CEH despite the lack of prior breast surgery. The large tumor size of the presumed CEH and its tendency for rapid growth made us attempt to treat the breast lesion with lumpectomy. Frozen section, however, revealed malignant cells in the hard part of the capsule, leading to the conversion of breast surgery from lumpectomy to nipple-preserving mastectomy. Postoperative pathological study showed that the tumor was composed of atypical cells growing in cribriform, tubular, and papillary fashions. In addition to the malignant cells, cyst wall of the CEH had abundant fibrosis and hemosiderin deposits. This is the first CEH case after no prior breast surgery, which had breast cancer within it. When a breast CEH is suspected, careful follow-up is imperative to avoid underestimating a possible concomitant breast cancer.