Where and how frequently p53 abnormalities are involved in the development of pleomorphic adenoma (PA) and its malignant progression to carcinoma was investigated. The presence of p53 gene abnormalities was analyzed in eight patients with carcinoma in pleomorphic adenoma (CPA) by polymerase chain reaction (PCR)-based assays and immunohistochemistry. Normal salivary gland tissue, adenomatous, transitional and carcinomatous areas were microdissected from archival microslides and analyzed for allelic deletions of the p53 gene using two microsatellite markers at the p53 locus; dinucleotide (CA)n repeat and pentanucleotide (AAAAT)n repeat. Loss of heterozygosity (LOH) of the p53 gene was detected in 57% of adenomas, 86% of transitional lesions and 86% of carcinomas. In contrast, overexpression of p53 oncoprotein was noted immunohistochemically in 13% of adenomas, 50% of transitional areas and 75% of carcinomas. All of the tumors with immunoreactivity for p53 oncoprotein demonstrated LOH. Moreover, when LOH was present in adenomatous or transitional areas, the identical LOH was always detected in the corresponding carcinomatous areas in the same CPA tumors. These findings indicate that p53 gene mutation is an early event and occurs frequently at an early stage of precancerous lesions and may be responsible for most cases of malignant transformation of PA.