Alterations in the p53 tumor suppressor gene are the most frequent genetic abnormalities in human cancers. The p53 protein is present in normal cells, and is assumed to induce G1 arrest or apoptosis in the presence of DNA lesion. The mutant protein lacks this property. Squamous cell carcinomas of the head and neck (SCCHN) are related to carcinogens in tobacco and alcohol, and provide a good model of multiple-step carcinogenesis in association with DNA damage and p53-related tumorigenesis. Stabilization of the mutant p53 protein allows immunohistochemical analyses (IHC) to be routinely used to demonstrate the mutant p53 protein in tissue samples, whereas normal p53 protein is undetectable. Ninety-nine squamous cell carcinomas, 8 in situ carcinomas, 31 preneoplastic lesions and 79 normal carcinogen-exposed mucosas of the head and neck from a total of 107 patients were examined for the expression of p53 tumor suppressor gene protein. Samples were collected before treatment, and stained with p53 specific mono- and polyclonal antibodies (DO-7, Pab 1801 and 240, CM1) using an indirect immunoperoxidase technique. Proliferating cell nuclear antigen (PCNA) provided semiquantitative estimates of proliferation. The main localizations were the pharynx (64/107) and the larynx (21/107). Positive IHC detection of p53 was observed in 9% of normal-appearing carcinogen-exposed mucosas, 37% of hyperplasias, 68% of dysplasias, 75% of in situ carcinomas, and 56/99 (56.5%) of primary tumor samples. Mucosas from 15 control patients under 10 years of age were negative. There was no correlation between p53 IHC and localization, differentiation or TNM staging.(ABSTRACT TRUNCATED AT 250 WORDS)