Three groups of metaplastic mammary carcinoma are distinguished: squamous cell carcinoma, invasive ductal carcinoma with partial squamous metaplasia and carcinoma with formation of heterologous elements. Squamous cell carcinoma is subdivided into 5 variants: solid and spindle cell variants, carcinoma in preexisting cyst and fibroadenoma; carcinoma resulting from total squamous metaplasia of invasive ductal carcinoma. Squamous cell mammary carcinoma has a relatively favourable prognosis. Partial squamous metaplasia has minimal influence on the course of the disease.