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Purpose This randomized trial compared outcomes of passive scattering proton therapy (PSPT) versus intensity-modulated (photon) radiotherapy (IMRT), both with concurrent chemotherapy, for inoperable non-small-cell lung cancer (NSCLC). We hypothesized that PSPT exposes less lung tissue to radiation than IMRT and thereby reduces toxicity without compromising tumor control. The primary end points were grade ≥ 3 radiation pneumonitis (RP) and local failure (LF). Patients and Methods Eligible patients had stage IIB to IIIB NSCLC (or stage IV NSCLC with a single brain metastasis or recurrent lung or mediastinal disease after surgery) who were candidates for concurrent chemoradiation therapy. Pairs of treatment plans for IMRT and PSPT were created for each patient. Patients were eligible for random assignment only if both plans satisfied the same prespecified dose-volume constraints for at-risk organs at the same tumor dose. Results Compared with IMRT (n = 92), PSPT (n = 57) exposed less lung tissue to doses of 5 to 10 Gy(RBE), which is the absorbed Gy dose multiplied by the relative biologic effectiveness (RBE) factor for protons; exposed more lung tissue to ≥ 20 Gy(RBE), but exposed less heart tissue at all dose levels between 5 and 80 Gy(RBE). The grade ≥ 3 RP rate for all patients was 8.1% (IMRT, 6.5%; PSPT, 10.5%); corresponding LF rates were 10.7% (all), 10.9% (IMRT), and 10.5% (PSPT). The posterior probability of IMRT being better than PSPT was 0.54. Exploratory analysis showed that the RP and LF rates at 12 months for patients enrolled before versus after the trial midpoint were 21.1% (before) versus 18.2% (after) for the IMRT group (P = .047) and 31.0% (before) versus 13.1% (after) for the PSPT group (P = .027). Conclusion PSPT did not improve dose-volume indices for lung but did for heart. No benefit was noted in RP or LF after PSPT. Improvements in both end points were observed over the course of the trial.