Metaplastic carcinoma accounts for only 1% of all breast cancer. Low-grade adenosquamous carcinoma (LGASCa) of the breast belongs to the family of metaplastic carcinomas. Here, we report on a case of LGASCa coexisting with sclerosing adenosis (SA). The patient was a 66-year-old woman. Eight years previously, she had an excisional breast biopsy in a hospital and was diagnosed with SA of her left breast. Three years ago, she presented to our hospital with deformity of the left nipple. We performed a core needle biopsy (CNB) and diagnosed SA. One year ago, a rebiopsy by CNB was performed because of the appearance of cysts in her left breast and worsening of the deformity of the left nipple. The diagnosis of the CNB was SA again. A third CNB was performed 4 months ago because the physical findings suggested malignancy; however, the CNB specimen was diagnosed as SA again. We decided to perform total mastectomy for the final diagnosis. Gross examination of the mastectomy material showed multiple cysts, the largest of which measured 3.3 × 2.2 × 2.0 cm. Histologically, the tumor was an irregularly shaped mass with duct proliferation and multiple cysts. The infiltrating tumor cells were scattered at the tumor periphery, and some infiltrating tumor cells resembled squamous cells. The tumor cells consisted of two types: one cell type was immunopositive for cytokeratin(CK)14 and p63, and the other cell type was immunopositive for CK14 and immunonegative for p63. Based on the histological and immunohistochemical findings, the diagnosis was LGASCa coexisting with SA and multiple cysts of the breast.