Abstract Background Clinician reporting of symptomatic adverse events (AEs) in phase I trials uses the Common Terminology Criteria for Adverse Events (CTCAE). The utility of the patient-reported outcomes (PROs) version of the CTCAE (PRO-CTCAE) in this setting is unknown. This prospective, observational study compared patient- and clinician-reported symptomatic AEs in phase I patients. Methods Phase I study–eligible patients at Princess Margaret were surveyed with the PRO-CTCAE full-item library (78 symptomatic AEs) at baseline (BL), mid-cycle 1, and mid-cycle 2 (C2). Patient and trial characteristics, best response, and survival data were collected. Presence or absence of patient- (PRO-CTCAE) or clinician-reported symptomatic AEs were compared (kappa) at defined timepoints and overall (BL+ mid-cycle 1 + C2). Results Of 292 patients approached from May 2017 to January 2019, a total of 265 (90.8%) were consented, with 243 (91.7%) evaluable and 552 PRO-CTCAE surveys (completion rate = 98.7%) included in analyses. Evaluation of overall patient-reported symptomatic AEs identified 50 PRO-CTCAE and 11 CTCAE items with 10% or greater reporting frequency. Nineteen CTCAE items were reported as 1% or less despite matched PRO-CTCAE items reporting as 10% or greater. Underreported categories included sexual health, bodily emissions, and cognition. Clinician- relative to patient-reporting frequency (ratio) demonstrated 9 symptomatic AEs with a 50-fold or more lower clinician reporting rate. Overall patient–clinician agreement for individual symptomatic AEs ranged from poor (κ = 0.00-0.19) to moderate (κ = 0.40-0.59), with discordance driven by lack of clinician reporting. Dyspnea (κ = 0.54) and peripheral neuropathy (κ = 0.63) at BL and limb edema (κ = 0.55) at C2 demonstrated the highest patient–clinician agreement. Conclusions Poor to moderate patient–clinician agreement for symptomatic AEs suggests clinician underreporting in phase I trials. Analyses of severity and interference PRO categories are ongoing.