Advances in tumor biology and clinical trials indicate that p53 transfer is an alternative therapy for head and neck squamous cell carcinoma. The aim of this study is to evaluate the biologic activity of multiple intraepithelial injections of Ad-p53 in patients with dysplastic oral leukoplakia, which is the most common premalignant lesion of the oral squamous cell carcinoma.From 2006 to 2007, 18 Chinese patients clinically and histopathologically diagnosed as dysplastic oral leukoplakia were recruited for this study. On a 15-day cycle, intraepithelial injections of Ad-p53 were administered once every three days at dose levels dependent upon lesion size/dose escalation sequence (1 x 10(8) vp). 24-48 h after the last injection, incisional biopsy were performed, and immunohistochemistry was used to examine the protein expression of P53 and P21(CIP/WAF).In the postreatment patients, P53 protein and P21(CIP/WAF) protein expression were significantly enhanced (100%, 89.9%, respectively) and statistical analysis revealed the expression of P53 protein had a positive correlation with that of P21(CIP/WAF) protein (r=0.598, P<0.01).Intraepithelial injections of gendicine is biologically active in patients with dysplastic oral leukoplakia. It may be a promising treatment for oral leukoplakia.