OBJECTIVE To determine immunohistochemical features and correlations between M1/M2 polarisation status with disease severity of post-caesarean scar diverticulum (CSD). METHODS Histological and immunohistological stainings were performed and inflammatory (CD16, CD163 and tumour necrosis factor-α (TNF-α)), fibrosis (α-smooth muscle actin (α-SMA)) and angiogenic (CD31) markers were examined in uterine tissues collected from patients with uterine scar diverticula (CSD) (n=37) and caesarean section (CS) (n=3). RESULTS CSD tissues have higher expression of α-SMA, TNF-α, CD16 and CD31 and lower expression of CD163 than CS tissue (p<0.05). Compared with adjacent tissues, thick-walled blood vessels, glands and fibrotic sites have higher expression of α-SMA, TNF-α and CD16. Statistical correlation was observed between the expression of CD16 and TNF-α (R=0.693, p<0.001), α-SMA (R=0.404, p<0.05) and CD31 (R=0.253, p<0.05) in CSD tissues, especially with the ratio of CD16/CD163 (R=0.590, p<0.01). A more significant difference was observed between the expression of CD16/CD163 and α-SMA (R=0.556, p<0.001), TNF-α (R=0.633, p<0.0001) and CD31 (R=0.336, p<0.05). CONCLUSIONS In this study, TNF-α, α-SMA, CD16 and CD31 proteins were overexpressed in all CSD cases, and CD16/CD163 was positively correlated with tissue inflammation, fibrosis and neovascularisation. Abnormal mononuclear macrophage infiltration may be involved in the origin and progression of CSD.