Despite medical advancements to combat pediatric malignancies, childhood cancer remains the leading disease-related cause of death among children in North America (1,2). Approximately 3.0% of anti-cancer interventions that enter clinical testing receive FDA approval for pediatric malignancies; 4.0% of oncology drugs already approved for adults are eventually approved and repurposed for pediatrics. These low success rates underscore the magnitude of challenges associated with developing safe and effective treatments for childhood cancer. When a particular cancer intervention is being developed for both adults and children, at least some data are gathered from studies with adults prior to beginning studies with children (3). Some argue that the lag time between first-in-human and first-in-child oncology studies is too long and impedes pediatric cancer patients’ access to novel cancer treatments (4). However, studies are carried out in adults prior to evaluation in pediatrics to minimize risk exposure to young trial participants. The optimal timing for testing of drugs in pediatric populations entails important trade-offs between harm and benefit. On the one hand, delayed testing in children is likely to spare pediatric research subjects from exposure to a drug that ultimately proves problematic due to safety, efficacy or pharmacology. On the other hand, delayed testing may deprive children access to potentially effective treatments. The time lag between initial adult and pediatric studies in oncology, its relationship with successful translation, and the trade-offs associated with current timing of testing has not, to our knowledge, been well defined. Neel et al. systematically analyzed this time lag for drugs that received regulatory approval for cancer; they found that pediatric studies lag adult studies by approximately 6.5 years (5). However, this study only looked at approved drugs, ignoring potential burdens of testing flagging drugs in pediatric populations. Nor did Neel et al. examine the risk and/or benefit for children’s participation in these trials, or an overall success rate for achieving regulatory approval for pediatric use. The present study aims to offer a systematic account of timing of pediatric cancer drug testing relative to adult testing, using a cohort of anti-cancer drugs and biologics identified based on their first pre-license efficacy trial. We will evaluate trade-offs in current research practices and compare risk and benefit of early vs. late pediatric cancer trial initiation.