BACKGROUND: A study was undertaken to evaluate the clinical outcome of particle therapy for stage I nonsmall cell lung cancer (NSCLC). METHODS: From April 2003 to April 2007, 80 patients with stage I NSCLC were treated with proton therapy or carbon‐ion therapy (57 with proton therapy and 23 with carbon‐ion therapy) using 3 treatment protocols. In the first protocol, 80 gray equivalents (GyE) of proton therapy was given in 20 fractions, and the second proton therapy protocol used 60 GyE in 10 fractions. For carbon‐ion therapy, 52.8 GyE was given in 4 fractions. After achieving promising preliminary results for the first protocol, the authors started to use the second proton therapy protocol to shorten the overall treatment time. Carbon‐ion therapy was started in 2005, and thereafter, both proton and carbon‐ion therapy plans were made for each patient, and the 1 that appeared superior was adopted. Patient age ranged from 48 to 89 years (median, 76 years). Thirty‐seven patients were medically inoperable, and 43 refused surgery. Forty‐two patients had T1 tumors, and 38 had T2 tumors. RESULTS: The median follow‐up period for living patients was 35.5 months. For all 80 patients, the 3‐year overall survival, cause‐specific survival, and local control rates were 75% (IA: 74%; IB: 76%), 86% (IA: 84%; IB: 88%), and 82% (IA: 87%; IB: 77%), respectively. There were no significant differences in treatment results among the 3 protocols. Grade 3 pulmonary toxicity was observed in only 1 patient. CONCLUSIONS: Proton therapy and carbon‐ion therapy are safe and effective for stage I NSCLC. Further investigation of particle therapy for stage I NSCLC is warranted. Cancer 2010. © 2010 American Cancer Society. The authors report 3‐year results of particle therapy for stage I nonsmall cell lung cancer at the Hyogo Ion Beam Medical Center, which is the only facility in the world providing both proton and carbon‐ion therapy. Despite inclusion of large T2 tumors, their results were comparable to published results of x‐ray stereotactic radiotherapy, which is usually used to treat smaller tumors.