We examined the capacity of children, adolescents, and young adults to assent and consent to participation in biomedical research, and what physician-investigators believe is important for patients in these age groups to know about such participation. The sample included 44 male and female subjects, ranging in age from 7 to 20 years, who were hospitalized to treat either pediatric cancer or obesity. The participants completed a structured interview that assessed knowledge of research participation using the elements outlined in the federal guidelines for informed consent. The study subjects were most knowledgeable about those elements of consent that assessed concrete information (e.g., freedom to ask questions, time elements involved, and the benefits of participation). They were less knowledgeable about those elements of informed consent that assessed abstract information (e.g., scientific vs therapeutic purpose of the study, and alternative treatments). Chronologic age was not related to knowledge of the elements of informed consent. The strategies that the study subjects used to reason about participation in research appeared to parallel their reasoning about other physical phenomena.