Pathological diagnosis fails in some pulmonary tumors, although they may be highly suspected to be primary lung cancer. We studied the outcome of stereotactic body radiotherapy for a clinically diagnosed primary stage I lung cancer without pathological confirmation.The current study included 37 patients (39 lesions) treated with stereotactic body radiotherapy who were clinically diagnosed with primary stage I lung cancer between August 1998 and April 2009 at our hospital. Pulmonary tumors were highly suspected to be malignant from physical and imaging examinations. Biopsies were performed for 62 % of patients, although malignancy was not pathologically confirmed. In the other 38 % of patients, a biopsy was not feasible. Median age of the patients was 77 years. Median tumor diameter was 20 mm. A total median dose of 48 Gy was prescribed to the isocenter in four fractions. Median follow-up period was 39 months.The 3-year overall survival, local control, and regional-distant control were 74.2, 94.0, and 68.6 %, respectively. In patients with tumors a parts per thousand currency sign20 mm, overall survival and regional-distant control were significantly higher than in patients with tumors > 20 mm (p a parts per thousand currency sign 0.001), whereas no significant difference was observed regarding local control. No grade 3-5 adverse events possibly, probably, or definitely related to the treatment were observed.Stereotactic body radiotherapy is safe and effective for a clinically diagnosed primary stage I lung cancer when pathological diagnosis is difficult even with repeat biopsies, or a biopsy is not feasible for reasons of the patient's health condition or wishes.