The p53 gene mutation pattern was used as a diagnostic marker of multiple and second primary lung carcinomas. Nine cases of multiple carcinoma, which were suspected clinicopathologically to be double or triple primary carcinomas. were examined for p53 protein expression by immunohistochemistry and for genetic abnormality of the p53 gene by polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) analysis. Nine tumors from four cases gave a positive result upon both immunostaining for the p53 protein and PCR-SSCP analysis of the p53 gene. These nine tumors showed different mobility shifts between exons 5 and 8. The four cases were diagnosed genetically as multiple primary carcinomas. To confirm the results of PCR-SSCP analysis, five tumors from two cases that showed different mobility shifts were further analyzed for their nucleotide sequences, and it was found that all of them had point mutations at different codons in exons 5 and 8. These findings suggest that the p53 gene mutation pattern is an effective marker for diagnosis of tumor multiplicity.