To reveal heterogeneous properties of triple-negative (TN) breast cancers (estrogen receptor negative, progesterone receptor negative and HER2 negative) and to clarify whether the developmental pathways to TN breast cancer are single or multiple, we conducted clinicopathological and immunohistochemical studies on TN breast cancers, with special reference to comparison of the invasive component (iIC) and the ductal component (dcIC) of invasive TN breast cancer and pure TN ductal carcinoma in situ (TNDCIS). Tumor tissues were obtained from 97 patients with TN invasive carcinoma and 10 patients with TNDCIS. Two histological subclassifications, “atypical” medullary carcinoma (type A, = 16) and carcinoma with a central acellular zone (type B, = 11), were distinguished from conventional ductal carcinoma. Other invasive ductal carcinomas were classified as type C (= 64) and special types were classified as type D (= 5). The follow-up period for the 96 patients ranged from 5 to 147.8 months (mean, 47.6 months). Out of 97 cases, dcIC was present in 29 (30%) cases and type A and B had significantly few ductal components, 0% and 18%, respectively. There were only six (6%) cases with non-TN cells in dcIC and TN cells in iIC and five of them were type C. In 13 (13%) cases, epidermal growth factor receptor (EGFR) expression existed only in iIC. Therefore, most of the TN carcinoma develops originally and rapidly invades at the early stage, especially in types A and B. The relapse rate of type B was the highest (36.4%) and the overall survival of patients with type B was the shortest (= 0.02), which indicates that the prognosis of type B is significantly worse than the other types. (Cancer Sci 2011; 102: 656–662)