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The cytological and histopathological features of a case of low-grade adenosquamous carcinoma of the breast in a 47-year-old Japanese female was studied. The fine needle aspiration biopsy specimen of this case revealed proliferating ductal cells and atypical squamous cells in the same cell clusters. The resected left breast mass had a relatively well-circumscribed hard mass measuring 1.6 cm at its greatest dimension. Histologic examination demonstrated the coexistence of atypical squamous and ductal components in the same lesion, with transition between these two components. Desmoplastic stromal reaction was observed. The majority of ductal components had a two-layered composition, and sclerosing adenosis-like lesions were also observed at the periphery of the tumor. Based on these cytologic and histological findings, this tumor was diagnosed as low-grade adenosquamous carcinoma of the breast. This is the first report of the cytological findings of low-grade adenosquamous carcinoma of the breast, and it is important for pathologists to consider the possibility of low-grade adenosquamous carcinoma when the cytological findings described above are present in an aspiration cytology specimen of the breast.