A primary squamous cell carcinoma of the breast was evaluated by fine needle aspiration cytology in a 60 year-old female. Squamous cell carcinoma is a rarely encountered lesion in the breast and nine cases of cytologic findings of mammary squamous cell carcinoma have been reported in the world literature. Our case appears to be one of these rare pure squamous cell tumors. The cytologic diagnosis was possible because of the well defined characteristics of the malignant squamous cells, similar to those found in other locations. Thorough examination of the patient did not reveal squamous cell carcinoma elsewhere.