Background: Stereotactic radiation therapy is a promising treatment for early stage lung cancer occurring in medically inoperable patients (pts). Robotic radiosurgery with the CyberKnife allows for real-time tumor tracking under free breathing conditions. It could reduce toxicity while maintaining the same efficacy. We report our initial experience with 51 pts treated with the fiducial-less Xsight Lung Tracking System. Methods: Patients were accrued after evaluation by thoracic surgeons and oncologists. Selection criteria were: single T1 or T2 pulmonary tumor, tumor size between 15 and 60 mm, N0 and M0. Initial staging included CT-Scan with contrast agent and FDG-PET. If an anatomopathological proof could not be obtained, treatment was proposed for evolutive lesions: increase in size on two consecutive CT-Scans and single FDG uptake of the tumor on PET. No chemotherapy was used until disease progression. Response was evaluated with the modified RECIST criteria every 3 months and toxicity with the CTCAE v4.0 grading scale. Results: 43 men (84%) and 8 women (16%) were treated for 25 T1 (49%) and 21 T2 (41%) lung cancer. 5 pts (10%) were treated for a relapse after prior surgery (4%) or radiation therapy (6%). Median follow-up was 15 months (5-30). Median age was 69 (50-85). All pts were smokers. Median tumor size was 24 mm (15-70). Histology was known for 19 pts (37%): 4% adenocarcinoma, 19% squamous, 8% large cell and 6% not otherwise specified. Median delivered dose was 60 Gy (36-60 Gy) in 3 fractions. Median growth tumor volume (GTV) was 11 cm³ (1.6-115). Local control rate was 86%, including 16 complete responses, 16 partial responses and 12 stable diseases. 7 progressions were observed. 1 pt relapsed with mediastinal lymph nodes and 6 pts with distant metastases and subsequently received chemotherapy. Disease-specific survival rate was 98% at 15 months. 7 grade 1 (14%) and two grade 2 (4%) radiation pneumonitis were found on CT-Scan at 3 months and disappeared on later controls. Conclusions: Local control rate was similar to what is reported in other studies using fiducials for tumor tracking. Toxicity seems lower. This method could represent a completely non-invasive curative treatment for pts not amenable to surgery.