Background: More pediatric drug trials are needed, but although specific pediatric regulations warrant safety, recruitment of children for these trials remains one of the main difficulties. Therefore, we investigated potential determining factors of nonparticipation in clinical research, in order to optimize research participation of children by recommending improved recruitment strategies. Methods: Between 1 January 2012 and 1 January 2014, we performed a prospective study among161 pediatric patients, aged 6 to 18 y, who were eligible for clinical research. We quantitatively analyzed the association of potential explanatory variables (e.g., age, cognitive development, experience, ethnicity) with nonparticipation and qualitatively analyzed interviews on reasons for nonparticipation. results: Sixty percent of the children did not participate in the research project on offer (39% decided not to participate, 21% were indecisive). Lower age, less disease experience, and less complex research with lower risk were predictive for not participating. Time constraint and extra burden were expressed as decisive reasons for not participating. conclusions: Strategies to optimize research participation should be aimed at younger children and their families, who are logistically challenged and unfamiliar with health care and research. Recommendations include informing pediatric patients and their families of the value of research; minimizing logistic burdens; and improving accessibility.