Abstract: Objective To retrospectively review the clinocopathological data of patients with breast ductal carcinoma in situ （DCIS）, and explore the distribution pattern of tumor-infiltrating lymphocytes （TILs） in DCIS patients complicated with microinfiltration. Methods Seventy-four DCIS patients were recruited in this study. Among them, 42 patients with pure DCIS （pure-DCIS） were assigned into the control group, and 32 cases of DCIS complicated with microinfiltration （mi-DCIS） were allocated into the experimental group. The clinicopathological data and TILs distribution were analyzed. Moreover, the TILs density between the DCIS area and microinfiltration area were statistically compared in the mi-DCIS group. Results Under 400× microscope, the hot spot and average TILs percentages were 57.50（18.75, 80.00）% and 8.00（5.00, 15.00）% in the mi-DCIS group,and 1.00（1.00, 5.00）% and 5.00（1.00, 16.25）% in the pure-DCIS group, respectively. Regardless of the hot spot or average value, the TILs percentages in the mi-DCIS group were significantly higher than those in the pure-DCIS group （both P < 0.001）. In the mi-DCIS group, the average proportion of TILs in the micro-infiltrating lesions was 15.00（5.00, 50.00）%, significantly higher compared with 8.00（5.00, 15.00）% in the carcinoma in situ area （P = 0.004）. Conclusions Compared with pure-DCIS alone, the density of TILs distribution in mi-DCIS is significantly enhanced. In the mi-DCIS group, the distribution of TILs in the micro-infiltrating lesions tends to elevate compared with the overall distribution pattern.