We report two cases of lung cancer in the third decade of life. One patient was a 27-year-old man complaining of persistent cough and chest pain. He was referred for examination of an abnormal shadow found on chest radiography. A diagnosis of adenocarcinoma consisting of bronchial gland cells was made by bronchial biopsy. The clinical stage was T4N2M1. Although he was treated with cisplatin-based chemotherapy and gefitinib, he died of lung cancer. The other patient was a 26-year-old Chinese woman complaining of hemosputum. Since histologic examination at thoracotomy revealed lung cancer, left lower lobectomy and lymph node dissection were performed. Histological examination of the resected specimen revealed pleomorphic carcinoma. The pathological stage was pT2N2MX. Although the absolute number of young adults with lung cancer has been increasing, the relative incidence among all cases of lung cancer has been decreasing. In some cases, young adults with lung cancer have been misdiagnosed as having bronchial asthma or pulmonary tuberculosis.