A 61-year-old Japanese woman underwent a partial mastectomy for cancer of the right breast (pT1cN0M0, stage I). Eight months later, chest computed tomography revealed two small nodules in the left lower lobe (Segmentum basale laterale and Segmentum basale posterius; S9 and S10). She thereafter underwent partial pulmonary resections for both diagnostic and treatment purposes. The nodule of S10 was pathologically diagnosed to be primary lung cancer. The nodule of S9 was pathologically diagnosed to be poorly differentiated adenocarcinoma. The same pattern of the distribution of the p53 mutation was observed in the DNA samples of the S9 nodule and the treated breast cancer. We therefore finally diagnosed the S9 nodule to be metastatic pulmonary carcinoma. A mutation analysis of the p53 gene is thus considered to be a good modality for differentiating metastases from primary carcinomas of the lung.