Basal cell carcinoma (BCC) of the skin is the most common human cancer, but its molecular-genetic pathogenesis is unclear. In many other types of cancer, mutations of the tumor-suppressor gene p53 occur frequently and may lead to overexpression of a long-lived mutant form of p53 protein. In this study, overexpression of p53 protein was detected immunohistochemically in 30 (83%) of 36 specimens of BCC of the head and neck. The same regions of tumor typically were reactive both with a monoclonal antibody (PAb240) specific for the mutant protein and with one (PAb1801) directed against an epitope common to both wild-type and mutant p53 protein. Keratinocytes of chronically sun-exposed epidermis adjacent to BCCs also focally overexpressed p53 protein in the majority of cases, whereas those of sun-protected buttock skin did not. Mutation of p53 may form an important part of the pathogenetic sequence in a majority of cases of BCC.