Synchronous primary lung cancer (SPLC) occurs in up to 0.5% of patients with lung cancer. Among SPLC cases, coexistence of small cell carcinoma (SCLC) and non-small cell carcinoma has been reported in a very small fraction. Futhermore, there have been no reports discussing treatment and prognosis of SPLC presenting with SCLC and NSCLC. We report on two cases of SPLC presenting SCLC in limited stage and operable NSCLC. One patient developed synchronously SCLC and adenocarcinoma of the lung, while the other SCLC and squamous cell carcinoma of the lung. The clonal origin of these synchronous lung cancers was evaluated using immunohistochemical and polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analyses. Both of the patients were diagnosed based on transbronchial lung biopsy (TBLB) and mediastinoscopic biopsy. They were successfully treated with chemoradiotherapy and adjuvant surgery, and are now doing well without any signs of tumor progression for about one year. In both cases, a response of mediastinal lymph node for concurrent chemoradiotherapy was quite different from that of the mass in the lung field. In case 2, p53 mutation was observed in the SCLC tissue, but not in the NSCLC tissue by PCR-SSCP. In both cases, carcinoembryonic antigen was documented in the NSCLC tissue, but not in the SCLC tissue by immunohistochemical staining. This report indicates the importance of the accurate diagnosis of SPLC by employing TBLB and/or media-stinoscopy for the optimal treatment of patients having SPLC presenting with SCLC and NSCLC. Diagnostic criteria and standard treatment of this disease should be established.