We present a case with metachronous double lung cancers proved by molecular analysis. A male patient first had lung cancer discovered when he was 67 years old, and a second lung cancer was found when he was 71 years old. Pathologically, the first cancer was moderately to well differentiated adenocarcinoma and the second cancer was well differentiated adenocarcinoma. These findings suggested that the second cancer was not metastasis from the first lung cancer but a second primary lung cancer. According to the PCR and oligonucleotide mutation specific dot blot hybridization, the first lung cancer had a GGT to GTT mutation, while the second lung cancer had a GGT to TGT mutation at codon 12 of the K-ras gene. These results thus prove that these tumors were metachronous double cancers.