Several studies have shown that symptomatic toxicities associated with anti-cancer treatments, such as nausea and vomiting, are frequently under-reported by health care providers, even when prospectively collected within clinical trials. Such under-reporting can produce underestimation of the absolute rate of toxicity, which is highly relevant information for patients and their physicians in clinical practice, as well as for regulatory authorities. Systematic collection of patient-reported outcomes (PROs) has been demonstrated to be a valid, reliable, feasible, and more precise approach to tabulating symptomatic toxicities, compared to health care provider reporting. The U.S. NationalCancer Institute has developed a PRO version of the Common Terminology Criteria for Adverse Events (the "PRO-CTCAE") to improve toxicity measurement in clinical trials. The PRO-CTCAE is an item bank with 124 individually validated toxicity questions for patients, with an accompanying software system for directly collecting information about symptomatic adverse events from patients. The barriers and challenges that need to be addressed when considering broad integration of PRO toxicity monitoring in cancer clinical trials are discussed, including challenges related to data collection logistics, analytic approaches, and resource utilization. The advent of instruments like the PRO-CTCAE, conceived to describe therapy side effects from the patient perspective, brings the potential both to improve the quality of data for risk-benefit evaluation in clinical research, and to provide future patients facing treatment decisions with information about prior experiences of their peers.3