Background: Metformin is one of the most commonly prescribed drugs for type 2 diabetes, and some reports have suggested that metformin may reduce cancer risk. Diabetics treated with metformin have a 23% reduction in the risk of cancer, including breast cancer. In addition, it is reported that the breast cancer patients with metformin treatment for diabetes showed favorable prognosis compared with those without metformin treatment. However, the mechanism underlying the positive effects of metformin on cancer treatment remains unclear. Methods: We conducted a prospective study to evaluate the effect of preoperative metformin on early breast cancer patients. The patients took a daily dose of metformin orally for 7 to 21 days before surgery. We evaluated the effects on immunological factors (TILs, CD4+, CD8+, PD-L1 and ALDH1) by comparing core needle biopsies (CNB) obtained before surgery with surgical specimens. Results: Seventeen breast cancer patients were enrolled in this prospective study and administered metformin before surgery, during the period from January to December 2016. We analyzed 59 patients who received surgery during the same period as a control group. In the control group, there was no significant difference in TILs between CNB and surgical specimens (Rs = 0.63). In the metformin group, TILs were negative in CNB and surgical specimens of 15 (88%) and 8 (48%) cases, low in 2 (12%) and 8 (48%), and intermediate in 0 and 1 (6%), respectively. These TILs increases were confirmed in 5 (29%) patients (p = 0.09), while a decrease was confirmed in 2 (12%). The expressions of CD4+ and CD8+ by TILs were increased in 41% and 18% of surgical specimens, respectively. (p = 0.02, p = 0.09) However, there was no statistically significant difference in these immunological factors and PD-L1 or ALDH1 expression between before and after metformin. Conclusions: In our small cohort, preoperative metformin administration shows positive impact on CD4 positive lymphocytes significantly and has tendency of increasing both TILs and CD8 positive lymphocytes. However, we should keep in mind that our sample size is small. Further study is necessary to uncover the mechanisms of favorable effects of metformin on breast cancer patients.