To evaluate treatment safety in oncology trials, clinicians provide adverse event (AE) grades and attributions (i.e., relatedness to protocol treatment) using the National Cancer Institute’s (NCI’s) Common Terminology Criteria for AEs (CTCAE). However, clinicians generally underreport AEs relative to patients (Basch et al., 2006) and provide unreliable AE attributions (Le-Rademacher et al., 2017). To integrate the patient voice in AE reporting, the NCI developed the Patient-Reported Outcomes (PRO) version of the CTCAE (PRO-CTCAE), a library of 124 questions for creating custom forms for patients to report relevant symptomatic AEs. While PRO-CTCAE does not include attribution questions for patients, PRO-CTCAE should be administered prior to treatment initiation (i.e., at baseline) to facilitate analytical approaches for assessing treatment-relatedness. In this presentation, novel analytical approaches for between-arm comparisons and within-arm estimation of toxicity, which account for the baseline assessment, will be described and compared using PRO-CTCAE data from a prostate cancer trial. When accounting for preexisting symptoms, PRO-CTCAE better delineated between arms than CTCAE.