The National Cancer Institute's Symptom Management and Health-Related Quality of Life Steering Committee held a clinical trials planning meeting (September 2011) to identify a core symptom set to be assessed across oncology trials for the purposes of better understanding treatment efficacy and toxicity and to facilitate cross-study comparisons. We report the results of an evidence-synthesis and consensus-building effort that culminated in recommendations for core symptoms to be measured in adult cancer clinical trials that include a patient-reported outcome (PRO). We used a data-driven, consensus-building process. A panel of experts, including patient representatives, conducted a systematic review of the literature (2001-2011) and analyzed six large datasets. Results were reviewed at a multistakeholder meeting, and a final set was derived emphasizing symptom prevalence across diverse cancer populations, impact on health outcomes and quality of life, and attribution to either disease or anticancer treatment. We recommend that a core set of 12 symptoms--specifically fatigue, insomnia, pain, anorexia (appetite loss), dyspnea, cognitive problems, anxiety (includes worry), nausea, depression (includes sadness), sensory neuropathy, constipation, and diarrhea--be considered for inclusion in clinical trials where a PRO is measured. Inclusion of symptoms and other patient-reported endpoints should be well justified, hypothesis driven, and meaningful to patients. This core set will promote consistent assessment of common and clinically relevant disease- and treatment-related symptoms across cancer trials. As such, it provides a foundation to support data harmonization and continued efforts to enhance measurement of patient-centered outcomes in cancer clinical trials and observational studies.