Abstract
A 71-year-old man was diagnosed as having right primary lung squamous cell carcinoma, clinical stage IIIA, but he refused treatment. However, the right upper lobe nodule and lymph node (LN) #4R showed gradual shrinking without treatment. Four years after the diagnosis, a new nodule was detected in the left lung field. We considered that this new nodule might be metachronous primary lung cancer, and hence resected it for diagnosis and treatment. The tumor in the left lung was diagnosed as basaloid squamous cell carcinoma, and that in LN #4R was diagnosed as squamous cell carcinoma with keratinization. Therefore, the patient was diagnosed as having metachronous primary lung cancer that developed during the spontaneous regression of locally advanced lung cancer.