Objective: To analyze the clinicopathological features of type 2 diabetes mellitus complicated with colorectal cancer (DCRC). Methods: A case-control study was conducted. Inclusion criteria: (1) hospitalized patients receiving fibrocolonoscopy; (2) adenocarcinoma and mucinous adenocarcinoma diagnosed by pathology; (3) with preoperative cTNM clinical staging; (4) colorectal cancer patients undergoing surgical treatment; (5) with postoperative pTNM staging; (6) no smoking or drinking habits. Exclusion criteria: (1) familial adenomatous polyposis (FAP); (2) Lynch syndrome; (3) carcinoma of anal canal and perianal carcinoma; (4) multiple primary cancer; (5) with serious cardiocerebrovascular diseases or multiple organ failure. Clinicopathlogical data of 32 DCRC patients who were diagnosed and treated in Peking University Shougang Hospital from December 2017 to December 2018 were retrospectively collected and analyzed. Forty nondiabetic colorectal cancer (CRC) patients during the same period were selected as control group according to the sex ratio and the age difference less than 5 years. Student′s t test and χ² test were used to compare the difference between the two groups in baseline clinicopathological data, clinical test results, tumor markers and infiltration status of T cells in tumor immune microenvironment. Results: Among 32 DCRC patients, 24 were males and 8 were females with a mean age of (63.0±1.7) years; among 40 CRC patients, 30 were males and 10 were females with a mean age of (60.5±1.6) years. The duration of diabetes mellitus in DCRC patients (from the diagnosis of diabetes mellitus to the diagnosis of colorectal cancer) was (9.2±1.3) years. The body mass index (BMI) of DCRC group was significantly higher than that of CRC group [(24.8±0.6) kg/m² vs. (23.2±0.4) kg/m², t=2.372, P=0.020]. There were no significant differences in other baseline data (sex, age, primary site of tumor, R0 resection rate, pathological stage, pathological type, differentiation degree of tumor, preoperative intestinal obstruction) between the two groups (all P>0.05). Serum triglyceride level in DCRC group was higher than that in CRC group [(2.1±0.2) mmol/L vs. (1.5±0.1) mmol/L, t=3.085, P=0.003], while hemoglobin [(120.3±5.2) g/L vs. (132.7±2.8) g/L, t=-2.224, P=0.029], anti- thrombin III [(94.2±3.7)% vs. (103.5±2.4)%, t=-2.197, P=0.031], and red blood cell count [(4.2±0.1)×10¹²/L vs. (4.5±0.1)×10¹²L, t=-2.055, P=0.044] were all lower than those in CRC group. The preoperative carcinoembryonic antigen (CEA) level in DCRC group was higher than that in CRC group [(50.3±21.8) μg/L vs. (5.6±1.0) μg/L, t=2.339, P=0.022]. There were no significant differences in preoperative levels of other four tumor molecular markers (CA199, CA242, CA724 and CA125) between the two groups (all P>0.05). The expression of Foxp3 [specific markers of CD4+, CD25+ regulatory T cells (Treg)] in DCRC group was higher than that in CRC group [(82.7±6.2) cell/HPF vs. (62.6±4.9) cell/HPF, t=2.586, P=0.012]. There were no significant differences in the infiltration of CD4, CD8, PD-1 and PD-L1 positive cells between two groups (all P>0.05). Conclusions The average diabetic history of DCRC patients is nearly 10 years. They have higher BMI and serum CEA level, and more Treg cell infiltration in the tumor. Close attention should be paid to these patients in clinical practice.