Byline: Dominique Dupont, Department of Psychology, Montreal University, Quebec, Canada (Ms Dupont and Drs Beaudoin and Beauchamp); Sainte-Justine Hospital Research Center, Montreal, Quebec, Canada (Mss Dupont and Tran and Drs Beaudoin and Beauchamp); Department of Psychology, The Hospital for Sick Children, Toronto, Ontario, Canada (Dr DÈ¨sirÈ¨); Division of Pediatric Emergency Medicine, Montreal Children's Hospital, McGill University Health Center, Montreal, Quebec, Canada (Dr Gagnon); and School of Physical & Occupational Therapy, McGill University, Montreal, Quebec, Canada (Dr Gagnon).; Cindy Beaudoin; Naddley DÈ¨sirÈ¨; Maria Tran; Isabelle Gagnon; Miriam H. Beauchamp Abstract OBJECTIVE: To report preliminary empirical data on a novel, developmentally appropriate, observational postconcussive symptoms inventory for infants, toddlers, and preschoolers. SETTING: Emergency departments of 2 tertiary, urban pediatric hospitals. PARTICIPANTS: Ninety-eight children (0-8 years of age; mean age at injury = 33.00, SD = 24.7 months) with mild traumatic brain injury (concussion) divided into younger (0-2 years) and older (3-8 years) age groups. DESIGN: Observational study. MAIN MEASURE: The Report of Early Childhood Traumatic Injury Observations & Symptoms (REACTIONS) documents 17 postconcussive symptoms representing observable manifestations thereof and was completed by parents in the acute (24-48 hours; n = 65), subacute (7-14 days; n = 78), and/or persistent phase (25-35 days; n = 72) post-mild traumatic brain injury. RESULTS: Different patterns of postconcussive symptoms were reported by age group, with behavioral manifestations particularly salient in younger children. More children younger than 2 years had sleep and comfort-seeking symptoms at each of the 3 postinjury time points. CONCLUSION: Postconcussive symptoms may manifest differently after mild traumatic brain injury sustained during early childhood. To fully understand and address their presence and evolution, developmentally sound measures such as the REACTIONS inventory are required.