BACKGROUND: Symptoms arising from disease or treatment are subjective experiences. Insight into pediatric oncology treatment side effects or symptoms is ideally obtained from direct inquiry to the ill child. A concept-elicitation phase in a patient-reported outcome (PRO) instrument design provides an opportunity to elicit children's voices to shape cancer symptom selection and terminology. METHODS: Through semistructured, one-on-one, voice-recorded interviews, symptom data were collected from 96 children with cancer between the ages of 7 and 20 years who were undergoing oncologic treatment at 7 pediatric oncology sites in the United States and Canada. RESULTS: The mean number of symptoms reported per child over the prior 7 days was 1.49 (range, 0-7; median, 1; standard deviation, 1.56). The most common symptoms across all age groups were tiredness or fatigue, nausea or vomiting, aches or pains, and weakness. There was not a statistically significant correlation between self-reported wellness and the number of reported symptoms (r = -0.156, n = 65, P = .215) or the number of symptoms reported by age group or diagnosis type. Forty participants reported experiencing a change in their body in the past week, with one-third of these changes unanticipated. Only through direct questions about feelings were emotional symptoms revealed because 90.6% of interviewees who discussed feelings (48 of 53) did so only in the context of direct questioning on feelings. Adolescents were more likely than younger children to discuss feelings as part of the interview. CONCLUSIONS: Concept elicitation from children and adolescents has the potential to enable researchers to develop age-appropriate, accurately representative PRO measures.