Objective Juvenile polyposis syndrome (JPS) may present epithelial dysplasia, while the molecular mechanism of dysplasia has not been explained. This paper aims to study its pathologic mechanisms by detecting the expressions of proliferating cell muclear antigen (PCNA), P53 protein (DO-7), P21 wafl and P53 mRNA in JPS cases with dysplasia. Methods Thirty-five specimens from 5 cases with JPS (24 specimens with dysplasia and 11 without dysplasia), 5 normal specimens from colonic mucosa, 32 specimens from simple JP and 10 specimens from colorectal cancer were collected. The expressions of PCNA, P53 protein (DO-7) and P21 waf1 were detected by immunohistochemistry. The expression of P53 mRNA was detected using in situ hybridization techniques. Results The average PCNA labelling index, the positive expressions of P53 protein and P53 mRNA, and the positive expression areas of P53 in JPS cases with dysplasia were significantly higher than those in JPS cases without dysplasia. There was no significant difference in the positive expression rate of P21 waf1 between them. The expressions of P53 protein and P53 mRNA were positively correlated with PCNA expression in JPS cases with dysplasia. Conclusions PCNA and the variation of P53 gene may be involved in the formation of dysplasia. The genetic changes of P53 might play an important role in the induction of cell proliferation.