OBJECTIVE: This study is the first to assess posttraumatic stress disorder (PTSD) in young children with cancer and to examine individual, medical, and parent-relatedm determinants. METHODS: Forty-eight patients with cancer, ages 8-48 months, were assessed in an average of 15 months after their diagnosis, using mothers as informants to complete the Posttraumatic Stress Disorder Semi-Structured Interview and Observational Record for Infants and Young Children. Additionally, mothers and the attending pediatric oncologist completed questionnaires on various determinants of child PTSD. RESULTS: Nine children (18.8%) met the age-appropriate criteria for full PTSD proposed by Scheeringa et al., and 20 children (41.7%) met criteria for partial PTSD. Multivariate logistic regression analysis revealed that higher child age at diagnosis (odds ratio = 1.11) and maternal PTSD severity (odds ratio = 1.14) increased the risk of full or partial PTSD in children. CONCLUSIONS: This study provides evidence for a substantial prevalence of PTSD in young children with cancer and identifies important risk factors. Copyright © 2012 John Wiley & Sons, Ltd.