Abstract: Objective:When a patient presents with multiple pulmonary lesions with a similar malignant histology, it is a dilemma to decide the clonal origin of individual lesions and the tumor staging. However, disease stage is the most important factor for determining prognosis and treatment planning for lung cancer. We are able to identify the clonality using molecular biology technique in this report.Methods:Each lesion in 2 cases with multiple lung cancers was microdissected from paraffin slides and digested with proteinase K. Genomic DNA extracted was amplified by 2 sequential polymerase chain reactions (PCRs) with primers for exon 1 of K-ras, and for each exon of exon 5 to 9 of p53, and then followed by electrophoresed in a minigel for non-radioisotopic single-strand conformation polymorphism (SSCP) and visualized by silver staining. DNA sequencing was performed if necessary to confirm a mutation found upon SSCP analysis.Results:There were two malignant nodules in case 1, only one presented a mutation of K-ras in codon 12 of exon 1 (GGT→TGT); there were three malignant modules in case 2, one presented a mutation of K-ras in codon 12 of exon 1 (GGT→TGT),an other presented a mutation of p53 in codon 48 of exon 5 (GTG→TTG), none of mutation was in third one.Conclusion:The multiple lung cancers in both the cases were of different clonal origins. Therefore, it is recommended that the detection of the point mutations of K-ras and p53 by PCR-SSCP is an effective method to differentiate the clonal origins of multiple malignant nodules of the lung.