Stereotactic body radiation therapy is a new treatment modality for early-stage non-small-cell lung cancer, and is being intensively investigated in the United States, the European Union, and Japan. We started a feasibility study of this therapy in July 1998, using a stereotactic body frame. The eligibility criteria for primary lung cancer were: (1) solitary tumor less than 4 cm; (2) inoperable, or the patient refused operation; (3) histologically confirmed malignancy; (4) no necessity for oxygen support; (5) performance status equal to or less than 2, and (6) the tumor was not close to the spinal cord. A total dose of 48 Gy was delivered in four fractions in 2 weeks in most patients. Lung toxicity was minimal. No grade II toxicities for spinal cord, bronchus, pulmonary artery, or esophagus were observed. Overall survival for 29 patients with stage IA, and 14 patients with stage IB disease was 87 % and 80 %, respectively. No local recurrence was observed in a follow-up of 3-50 months. Regional lymph node recurrence developed in 1 patient, and distant metastases developed in 4 patients. We retrospectively analyzed 241 patients from 13 Japanese institutions. The local recurrence rate was 20% when the biological equivalent dose (BED) was less than 100 Gy, and 6.5% when the BED was over 100 Gy. Overall survival at 3 years was 42% when the BED was less than 100 Gy, and 46% when it was over 100 Gy. In tumors which received a BED of more than 100 Gy, overall survival at 3 years was 91% for operable patients, and 50% for inoperable patients. Long-term results, in terms of local control, regional recurrence, survival, and complications, are not yet evaluated. However, this treatment modality is highly expected to be a standard treatment for inoperable patients, and it may be an alternative to lobectomy for operative patients. A prospective trial, which is now ongoing, will, answer these questions.