Objective We identified factors influencing outcomes in patients with medically inoperable early stage lung cancer (MIESLC) treated with stereotactic ablative radiation therapy (SABR) at 14 centers in Turkey. Materials and Methods We retrospectively analyzed 431 patients with stage I-II MIESLC treated with SABR from 2009 through 2017. Age; sex; performance score; imaging technique; tumor histology and size; disease stage radiation dose, fraction and biologically effective dose with an alpha/beta ratio of 10 (BED10); tumor location and treatment center were evaluated for associations with overall survival (OS), local control (LC) and toxicity. Results Median follow-up time was 27 months (range 1-115); median SABR dose was 54 Gy (range 30-70) given in a median three fractions (range 1-10); median BED(10)was 151 Gy (range 48-180). Tumors were peripheral in 285 patients (66.1%), central in 69 (16%) and <1 cm from mediastinal structures in 77 (17.9%). Response was evaluated with PET/CT in most cases at a median 3 months after SABR. Response rates were: 48% complete, 36.7% partial, 7.9% stable and 7.4% progression. LC rates were 97.1% at 1 year, 92.6% at 2 years and 91.2% at 3 years; corresponding OS rates were 92.6%, 80.6% and 72.7%. On multivariate analysis, BED10> 100 Gy (P = .011), adenocarcinoma (P = .025) and complete response on first evaluation (P = .007) predicted favorable LC. BED10> 120 Gy (hazard ratio [HR] 1.9, 95% confidence interval [CI] 1.1-3.2,P = .019) and tumor size (<2 cm HR 1.9, 95% CI 1.3-3,P = .003) predicted favorable OS. No grade 4-5 acute side effects were observed; late effects were grade <= 3 pneumonitis (18 [4.2%]), chest wall pain (11 [2.5%]) and rib fracture (1 [0.2%]). Conclusion SABR produced encouraging results, with satisfactory LC and OS and minimal toxicity. BED10> 120 Gy was needed for better LC and OS for large, non-adenocarcinoma tumors.