Abstract Objectives Aim of this retrospective multicenter observational study was to provide data on outcomes and prognostic factors in patients affected with stage I histologically confirmed NSCLC treated with Stereotactic Ablative Radiotherapy (SABR, or Stereotactic Body Radiotherapy, SBRT) outside clinical trials. Materials and Methods We analyzed a cohort of 196 patients with histological/cytological diagnosis of NSCLC. Median age at treatment was 75 years old; median tumor diameter was 2.48 cm, and median GTV 13.3 cc. One hundred fifty-five patients had stage IA disease (79.1%) and 41 patients stage IB disease (20.9%). Total doses ranged from 48 to 60 Gy in 3–8 fractions. Primary endpoints of the study were safety (acute and late toxicity) and efficacy (Local Control, Disease-Free Survival, Overall and Cancer-Specific Survival). Results Median follow-up time was 30 months. The percentage of grade ≥2 pulmonary toxicity was 3%, and the 30 and 60 days mortality rate was 0%. Local Recurrence-Free Survival was 89.7% at 3 years. Fifty-nine patients (30.1%) had at least one failure (local and/or nodal and/or distant), with a Disease-Free Survival (DFS) rate at 3 years of 65.5%. Overall Survival (OS) and Cancer-Specific Survival (CSS) rates were 68% and 82.1% at 3 years, respectively. Median time to any recurrence was 15 months, while median overall survival time was 54 months. At multivariate analysis, stage IB was the only variable associated to a decrease in DFS, OS and CSS (HR 2.77, p = 0.006; HR 2.38, p = 0.009; HR 4.06, p ≤ 0.001, respectively). A difference in survival according to stage was also evident at the log-rank test ( p ≤ 0.0001 for CSS and OS). Conclusion The results of the present study support the routine use of SABR for stage I NSCLC in a daily practice environment. The only prognostic factor that has been confirmed by our analysis was tumor stage (IA vs. IB).