Cells from breast cancers lacking hormone receptors (estrogen receptor [ER], progesterone receptor [PgR]) and human epidermal growth factor receptor (HER) 2 strongly express the cell proliferation marker Ki-67. However, the mechanisms of and stimulus signals involved in cell proliferation of this type of breast cancer are not well understood. The aim of the present study was to examine the characteristics of signal transduction in triple-negative (ER-, PgR-, and HER2-negative) breast cancers. For 44 tumor samples, western blotting analysis was conducted to examine the phosphorylation of HER2, external signal-regulated kinase (ERK)1 and -2 and Akt, and the immunohistochemical phenotypes of the samples with respect to ER and HER2 were also assessed. Phosphorylation of HER2 was detected in 4 of 15 immunohistochemically HER2-positive tumor samples (26.7%). ERK1/2 was more highly phosphorylated in triple-negative breast cancers. Phosphorylation of Akt kinase was significantly higher in triple-negative breast cancers. Triple-negative breast cancers are characterized by increased phosphorylation of Akt kinase. In the present study, we found for the first time that there is a population with a significantly activated Akt pathway in this type of breast cancer.