Most children with peripheral facial palsy will not have a cause identified. Although leukemia can cause facial nerve palsy, the magnitude of the risk is unknown and recommendations for investigations are variable. We are currently conducting a randomized, placebo-controlled trial of prednisolone for the treatment of Bell's palsy in children within the Paediatric Research in Emergency Departments International Collaborative emergency research network. In the course of the assessment for eligibility of the trial, from 644 acute-onset facial palsy presentations we identified 5 children with previously undiagnosed leukemia. We estimate the rate of leukemia in children with acute-onset facial palsy who present to emergency departments to be 0.6% (95% confidence interval 0.2% to 1.6%). In accordance with these cases, we suggest consideration of a screening CBC count for acute-onset peripheral facial palsy presentations in children before initiation of corticosteroid treatment.