The impact of childhood obesity on fentanyl PK is relatively unknown. We developed a population pharmacokinetic (PopPK) model using opportunistically collected samples from a cohort of predominately obese children receiving fentanyl per the standard of care. Use of an allometric relationship between weight and clearance was appropriate for describing the PK of intravenous fentanyl in our cohort. Our proposed model-derived continuous infusion strategy maximized the probability of achieving target steady-state concentrations in children of varying weights.