BACKGROUND Surgical resection has been the standard treatment for early-stage multiple primary lung cancer (MPLC). However, a significant proportion of patients with MPLC cannot undergo surgery. We explored here the role of stereotactic ablative radiotherapy (SABR) for patients with MPLC. METHODS We reviewed MPLC cases treated with SABR (50 Gy in 4 fractions or 70 Gy in 10 fractions) for the second tumor. Four-dimensional CT–based planning/volumetric image-guided treatment was used for all patients. Treatment outcomes/toxicities were analyzed. RESULTS For the 101 patients treated with SABR, at a median follow-up interval of 36 months and median overall survival of 46 months, 2-year and 4-year in-field local control rates were 97.4% and 95.7%. 2- and 4-year rates of overall survival (OS) were 73.2% and 47.5% and progression-free survival (PFS) were 67.0% and 58.0%. Patients with metachronous tumors had higher OS and PFS than did patients with synchronous tumors (2-year OS 80.6% metachronous vs. 61.5% synchronous; 4-year OS 52.7% vs. 39.7%; p=0.047; 2-year PFS 84.7% vs. 49.4%; 4-year PFS 75.6% vs. 30.4%; p=0.0001). For patients whose index tumor was treated with surgery or SABR, the incidence of grade ≥3 radiation pneumonitis was 3% (2/71), but this increased to 17% (5/30) for patients whose index tumor was treated with conventional radiotherapy. Other grade ≥3 toxicities included grade 3 chest wall pain (3/101, 3%) and grade 3 skin toxicity (1/101, 1%). CONCLUSIONS SABR achieves promising long- term tumor control, survival and could be a potential curative treatment of early-stage MPLC.