The clinical and pathologic features of 22 examples of pure ductal squamous cell carcinoma (SqCC) of the breast are reported. All patients were women with a mean age of 53 years. Only two of the 19 women with axillary dissection had metastases, but one woman had a positive supraclavicular lymph node in the presence of negative axillary nodes. All metastases were squamous carcinoma. All patients were eligible for a minimum of 5 years' follow-up; the cumulative 5-year disease-specific survival rate was 63%. Eight women developed metastases and one also developed local recurrence, all within 2 years, and all eight died from tumor within 4 years of initial therapy (mean, 2 years). Prophylactic postoperative radiation therapy and radiation for lymph node metastases at initial surgery did not prolong survival as four of the five recipients died from tumor. Although there were no statistically significant gross or histologic differences between recurrent and nonrecurrent neoplasms, larger neoplasms tended to metastasize. Immunohistochemical study for S-100 protein identified an immunoreactive cell component in five of seven neoplasms tested. This and light microscopic findings suggest myoepithelial proliferation, differentiation, or origin.