The frequencies of overexpression and mutation in the p53 tumor suppressor gene were examined in proliferative verrucous leukoplakia and oral squamous cell carcinoma with immunohistochemistry and single-strand conformation polymorphism analysis of DNA fragments amplified by polymerase chain reaction. Ten samples each of normal oral mucosa, proliferative verrucous leukoplakia, and squamous cell carcinoma were immunostained with antibodies against p53 protein; 8 of 10 cases of proliferative verrucous leukoplakia cases and 7 of 10 cases of oral squamous cell carcinoma were positive for p53 protein. Minimal staining was observed in normal oral tissues. The quantified labeling indexes demonstrated a range that corresponded to lesion progression. Single-strand conformation polymorphism analysis revealed p53 gene mutations within exons 5 to 8 in 40% (4 of 10) of the squamous cell carcinoma samples. Two of the 4 mutated squamous cell carcinoma samples lacked p53 expression. No p53 mutations were detected in proliferative verrucous leukoplakia tissues. Human papillomavirus 16 was identified in 2 of 7 p53 positive oral squamous cell carcinoma samples. Human papillomavirus 16 and 18 were identified in two of eight p53 positive proliferative verrucous leukoplakia samples. One p53 negative squamous cell carcinoma sample was positive for human papillomavirus 16 and had a mutation in exon 6 of the p53 gene. Human papillomavirus infection along with p53 expression plays a yet to be defined role in the pathogenesis of a limited number of cases of proliferative verrucous leukoplakia and squamous cell carcinoma. p53 immunohistochemistry, p53 gene mutations, and human papillomavirus infection prevalence do not provide a means to differentiate between leukoplakia and carcinoma and do not provide a predictive test for progression of leukoplakia to carcinoma.