To integrate the patient perspective into adverse event reporting, the National Cancer Institute developed a patient-reported outcomes version of the Common Terminology Criteria for Adverse Events (PRO-CTCAE). To assess the construct validity, test-retest reliability, and responsiveness of PRO-CTCAE items. A total of 975 adults with cancer undergoing outpatient chemotherapy and/or radiation therapy enrolled in this questionnaire-based study between January 2011 and February 2012. Eligible participants could read English and had no clinically significant cognitive impairment. They completed PRO-CTCAE items on tablet computers in clinic waiting rooms at 9 US cancer centers and community oncology practices at 2 visits 1 to 6 weeks apart. A subset completed PRO-CTCAE items during an additional visit 1 business day after the first visit. Primary comparators were clinician-reported Eastern Cooperative Oncology Group Performance Status (ECOG PS) and the European Organisation for Research and Treatment of Cancer Core Quality of Life Questionnaire (QLQ-C30). A total of 940 of 975 (96.4%) and 852 of 940 (90.6%) participants completed PRO-CTCAE items at visits 1 and 2, respectively. At least 1 symptom was reported by 938 of 940 (99.8%) participants. Participants' median age was 59 years; 57.3% were female, 32.4% had a high school education or less, and 17.1% had an ECOG PS of 2 to 4. All PRO-CTCAE items had at least 1 correlation in the expected direction with a QLQ-C30 scale (111 of 124, P<.05 for all). Stronger correlations were seen between PRO-CTCAE items and conceptually related QLQ-C30 domains. Scores for 94 of 124 PRO-CTCAE items were higher in the ECOG PS 2 to 4 vs 0 to 1 group (58 of 124, P<.05 for all). Overall, 119 of 124 items met at least 1 construct validity criterion. Test-retest reliability was 0.7 or greater for 36 of 49 prespecified items (median range intraclass correlation coefficient, 0.76 0.53-.96). Correlations between PRO-CTCAE item changes and corresponding QLQ-C30 scale changes were statistically significant for 27 prespecified items (median range r=0.43 0.10-.56; all P≤.006). Evidence demonstrates favorable validity, reliability, and responsiveness of PRO-CTCAE in a large, heterogeneous US sample of patients undergoing cancer treatment. Studies evaluating other measurement properties of PRO-CTCAE are under way to inform further development of PRO-CTCAE and its inclusion in cancer trials.