Objective:To investigate and analyze the characteristics of M2 macrophage infiltration and the clinical significance in patients with multiple primary cancers (MPCs) of head and neck in order to explore its role in the diagnosis and prognosis for patients with MPCs.Methods:RNA-seq data were downloaded from the Genomic Data Commons data portal (TCGA) and the R software v4.0.3 was used to statistically analyze the differences. A retrospective analysis was conducted by screening the clinical data of 44 patients (17 males and 27 females) with MPCs in head and neck from July 1998 to February 2016 in the Department of Oral and Maxillofacial Surgery, Peking University School and Hospital of Stomatology. Clinical data from a batch of 41 patients (28 males and 13 females) with gingival cancer and without MPCs from August 2013 to December 2015 were collected and analyzed. The number of CD163 positive cells and the expression patterns in immunohistochemically panoramic slices were observed under high magnification. Chi-square test and Spearman correlation analysis were used to compare the difference and correlation between the CD163 positive counts and/or depths of invasion and the number of incidences. The descriptive statistics on the clinical features was performed by SPSS 25.0.Results:TCGA database analysis showed that the infiltration of macrophage in patients with squamous cell carcinoma of head and neck (HNSCC) was increased compared to the para-cancer sites. A total of 142 tissue samples from 44 patients with MPCs were selected in the present single-center retrospective research. The number of CD163-positive cells in MPCs patients [90.9% (40/44)] was significantly increased compared to single gingival cancer patients [61.0%(25/41)] ( r=0.353, P=0.001), which was related to the number of occurrence ( r=0.368, P=0.001). The ratio of the CD163 counts in primary tumor to the depths of invasion was positively correlated with the number of onsets ( r=0.331, P=0.03). In terms of clinical features, the 44 patients with MPCs were mainly female, non-smoking, no alcohol addiction, no systemic history, Tis-T2 stage and N0 stage squamous cell carcinoma. The number of incidences ranged from two to eight. The incidence of cancer relative to synchronous cancer increased with the increased occurrence of MPCs. The primary cancer mainly occurred in tongue, gingiva and buccal sites, while the proportion of onset sites in gingiva, buccal and palate areas increased with the increased occurrence. Conclusions:M2 type macrophage counts and/or ratio to depth of infiltration were associated with the occurrence of MPCs, which could be used as a clinical indicator to distinguish single and MPCs in HNSCC. For early stage of HNSCC, patients with clinical characters of women, non-smoking, no alcohol addiction, no systemic medical history and sites of tongue, gingiva, and buccal should be paid more attention on their follow-up plan. The findings in the present study was also helpful to explore new treatment methods for the patients with MPCs.