Cost-effectiveness and burden-of-illness research of pediatric neurodevelopmental disorders (NDDs) requires accurate and comprehensive captures of health and non-health-sector costs and family out-of-pocket costs. The objective was to develop and validate the Resource Use Questionnaire—Preschool (RUQ–P) as a tool to capture intersectoral service use and family out-of-pocket costs in preschool children with an NDD from multiple-payer perspectives. The major categories of the RUQ–P include primary intervention, such as programs based on applied behavior analysis; childcare or school programs; other interventions, such as speech−language therapy or occupational therapy; other resources, such as medications, respite support, complementary and alternative medicine, and other services and treatments; productivity losses; and government subsidies and charitable gifts. The RUQ–P targeted preschool children ages 2–6 years with an NDD with completion by interview with the primary caregiver. It underwent pilot testing and was validated in a cohort of children in the Preschool Autism Treatment Impact study comparing early preschool behavioral interventions in Nova Scotia and New Brunswick, Canada. The RUQ–P collected data on resource use; service intensity; and out-of-pocket costs, such as hourly rates, costs of materials, and travel costs. A cost analysis was undertaken to determine whether the RUQ–P was responsive to differences between treatment groups and changes over time. Modifications were subsequently made to reduce respondent burden and missing data (i.e., more closed-ended and structured questions and removal of travel costs). Versions of the RUQ are being developed for 3 additional age groups, and the tool is being further validated in observational and experimental studies.