Lung cancer metastasis is typically determined by histologic similarity between distant and primary lesions. Herein, we present a 70-year-old Japanese woman with an adenocarcinoma in her lung and a squamous cell carcinoma in her femur; both tumors had an identical epidermal growth factor receptor mutation, G719 S. This indicated that both tumors had a common origin, despite their histologic dissimilarity. The tumor in the femur was thus identified genetically as a lung cancer metastasis. This case suggests that genetic analysis can determine whether a distant lesion is a lung cancermetastasis, particularly when the histology differs from that of the primary lesion. Lung cancer metastasis is typically determined by histologic similarity between distant and primary lesions. Herein, we present a 70-year-old Japanese woman with an adenocarcinoma in her lung and a squamous cell carcinoma in her femur; both tumors had an identical epidermal growth factor receptor mutation, G719 S. This indicates that both tumors had a common origin, despite their histologic dissimilarity. The tumor in the femur was thus identified genetically as a lung cancer metastasis. This case suggests that genetic analysis can determine whether a distant lesion is a lung cancermetastasis, particularly when the histology differs from that of the primary lesion.