A phase II trial of 5'-deoxy-5-fluorouridine (5'-DFUR), a new fluorinated pyrimidine analog which has been demonstrated to have potential superiority over 5-FU and tegafur for chemotherapy of murine tumors, was performed in patients with advanced non-small cell carcinoma of the lung and metastatic pulmonary tumors. 5'-DFUR at a dose of 800 mg/m2 was given per os every day for more than four weeks. None of 15 evaluable patients with non-small cell carcinoma of the lung and 15 evaluable patients with metastatic pulmonary tumors showed a complete or partial response. Toxic effects of 5'-DFUR included anorexia (29%), diarrhea (26%), nausea (23%), vomiting (10%), leukocytopenia (10%), general fatigue (10%), liver disorder (6%) and thrombocytopenia (6%).