Adenoid cystic carcinoma (ACC) is a rare type of breast carcinoma and this tumor makes up less than 0.1% of all mammary carcinomas; ACC is known to show a relatively favorable prognosis. Among a variety of microscopic growth patterns of mammary ACC, a solid variant is the rarest and this can cause diagnostic difficulties. We present here a case of a solid variant of mammary ACC that occurred in the right breast of a 40-year-old woman who was initially diagnosed with invasive ductal carcinoma. We discuss the histological and clinical characteristics of this case.