Accumulating evidence suggests that metformin reduces the incidence and mortality of colorectal cancer (CRC). However, the underlying mechanisms have not been fully clarified. The aim of this study is to examine the pathological characteristics of resected CRC from patients treated with metformin for type 2 diabetes mellitus (DM). A total of 267 patients with DM underwent curative colectomy for Stage I~III CRC and 53 (19.9%) patients had been treated medically including metformin. Pathological N-stage was significantly lower in metformin treated patients (p<0.05) with prolonged disease free survival (DFS) (p<0.05). Immunohistochemistry showed that the densities of CD3(+) and CD8(+) tumor infiltrating lymphocytes (TILs) in the invasive front area were significantly higher in 40 patients treated with metformin compared with propensity score matched cases without metformin (p<0.05). The density of tertiary lymphoid structures (TLS) in tumor stroma were markedly increased in metformin treated patients (p<0.001). In those tumors, there were more CD68(+) tumor associated macrophages (TAM) infiltrated (p<0.05), while the ratio of CD163(+) M2-phenotype was markedly reduced (p<0.001). Stromal fibrosis tended to be suppressed by metformin intake (p=0.051). These findings suggest that metformin drastically changes the characteristics of infiltrating immune cells in CRC and reprograms the tumor microenvironment from immunosuppressive to immunocompetent status, which may lead to suppression of microscopic tumor spread and improve the outcomes of patients with CRC and type 2 DM.This article is protected by copyright. All rights reserved.