ObjectiveTo summarize the incidence, patterns and laws of perineural invasion, and explore the path and the influencing factors of perineural invasion in hilar cholangiocarcinoma. MethodsA clinicopathologic study was conducted on sections from 52 patients with hilar cholangiocarcinoma to summarize the incidence and patterns of perineural invasion. The relationship of perineural invasion to lymph node metastasis, serum CA19-9, CEA, total bilirubin (TBIL) level, Bismuth-Corllet classification, or tumor penetration depth of bile duct walls was analyzed by association analysis. ResultsThe overall incidence of perineural invasion was 90.38% (47/52). However, the incidences of perineural invasion had no significant differences among various differentiated adenocarcinoma groups (P > 0.05). The incidences of perineural invasion were not correlated with the lymph node metastasis, serum CA19-9, CEA, TBIL level, and Bismuth-Corlette classification (P > 0.05), which was correlated with the tumor penetration depth of bile duct walls (P < 0.01). There were four patterns of perineural invasion, sequenced them according their incidences from high to low as follows: typeⅡ> typeⅢ> typeⅣ> typeⅠ. The pattern of perineural invasion was correlated with the degree of tumor differentiation (χ2=31.04, P < 0.01). ConclusionsThe incidence of perineural invasion is very high in hilar cholangiocarcinoma. The patterns of perineural invasion are similar in the same patient, and a variety of invasion patterns might coexist. While the pattern of perineural invasion is correlated with the degree of tumor differentiation. The incidence of perineural invasion is correlated with the tumor penetration depth of bile duct walls.