Objective: To observe the activity of the signaling pathway of phosphatidylinositol 3-kinase (PI3K)/Akt in the tumor tissues in obese breast cancer patients. Methods: Breast cancer specimens from 45 obese breast cancer patients (obesity group) and 37 normal-weight breast cancer patients (control group) were collected, and then, the PI3K mRNA expression was determined by RT-PCR, and the protein expressions of PI3K and phosphorylated Akt (p-Akt) were determined by immunohistochemical staining in the two groups of breast cancer tissues, respectively. Results: The results of RT-PCR showed that the PI3K mRNA expression was significantly elevated in breast cancer tissues of obesity group compared with control group (P<0.05). The results of immunohistochemical staining revealed that both PI3K and p-Akt protein expressions in breast tissues of obesity group were significantly higher than those in breast tissues of control group (both P<0.05) and, moreover there was a positive correlation between PI3K and p-Akt protein expression in either group of tissues (r=0.76 and r=0.81, both P<0.05). Conclusion: The activity of PI3K/Akt signaling pathway in cancer tissues of obese breast cancer patients is increased compared with non-obese breast cancer patients, thus, it is speculated to be one of the factors for the increased risk of breast cancer in obese women.