Background: Intensity-modulated radiation therapy (IMRT) has become the standard radiation delivery option for patients (pts) with localized (PCa) prostate cancer despite limited prospective data and increased cost relative to three-dimensional conformal radiation therapy (3D-CRT). The purpose of this study was to compare the treatment outcomes and toxicities of pts undergoing definitive dose-escalated radiation therapy using either 3D-CRT or IMRT for localized PCa. Methods: We identified men with clinically localized PCa treated with definitive external beam radiation at a single institution using >74 Gy between 1997-2013. Within each NCCN risk group, propensity score matching of 3D-CRT pts with one or two IMRT pts was performed accounting for age, race, diabetes, hypertension, Gleason score, T-stage, PSA, and hormone use. Conditional logistic and conditional Cox proportional hazards models were used to adjust for matched groups, with dose included as a separate covariate. Results: After matching,1,253 men (474 3D-CRT, 779 IMRT) were included. There were no significant differences between the IMRT and 3D-CRT pts according to clinical or treatment factors. There was no difference in biochemical failure (BCF), distant metastasis (DM), cancer specific survival (CSS), or overall survival (OS) amongst the low or intermediate risk subgroups. When examining the high-risk subgroup, pts receiving IMRT had higher rates of DM (HR 2.21 95% CI 1.04-4.74 p=0.040). There was a trend towards increased rates of BCF in pts receiving IMRT for high-risk disease (HR 1.60 95% CI 0.96-2.66 p=0.072). There was no difference in CSS or OS in the high-risk subset. In the intermediate risk group, patients receiving IMRT had increased rates of acute grade 3+ genitourinary toxicity versus 3D-CRT patients (OR 8.30 95% CI 2.13-2.15 p=0.002). There were otherwise no differences in acute or late grade 3+ genitourinary or gastrointestinal toxicity amongst IMRT versus 3D-CRT pts. Conclusions: Despite the widespread adoption of IMRT for localized PCa, our results do not demonstrate an improvement in outcomes or toxicity versus 3D-CRT. Further research is warranted to explain these findings.