Objective: To assess the treatment outcomes and impact of adjuvant chemotherapy on patients with early-stage medically inoperable lung cancer treated with stereotactic ablative radiosurgery (SABR). Material and Methods: The characteristics of 51 medically inoperable patients with lung cancer (T1–4 N0) who were treated with SABR and their response characteristics to the treatment were evaluated between June 2013 and June 2018. Results: The median patient age was 71 (range 48-86) years. Forty-three (84.3%) patients were men. Most patients had low-performance status (the Karnofsky Performance Scale 50-70), and 31 (60.8%) patients were aged above 65 years. The median tumor diameter was calculated as 32 (10-85) mm, and 29 (56.9%) patients had tumor diameters greater than 30 mm. The most common histology was squamous cell carcinoma (n=23, 45.1%). All patients received SABR treatment, and 12 (23.5%) patients received adjuvant chemotherapy. Disease-free survival could not be attained and overall survival (OS) was found at 32 (95% confidence interval, 21.2 to 42.8) months. Local recurrence was observed in only two (3.9%) patients. After SABR treatment, complete and partial responses were obtained in 35 (68.6%) and 15 (29.4%) patients, respectively. Significantly worse OS was observed in patients who received adjuvant chemotherapy compared to those who did not (16 and 40 months, respectively; and p=0.04). Conclusion: SABR treatment without chemotherapy could control the disease in individuals with low-performance status without any harmful side effects.