Little is known about the mechanism of pelvic injury in the pediatric population, an age range over which the pelvis undergoes tremendous structural change. We hypothesize that these structural changes influence pelvic fracture injury mechanisms. A probability sample of children under age 16 years in crashes were enrolled in an on-going crash surveillance system which links insurance claims data to telephone survey and crash investigation data. 15,725 children in side impact collisions were studied. Risk of pelvic fracture in side impact collisions was estimated and factors associated with these injuries were identified. Eight cases were examined using in-depth investigation to identify the injury mechanisms. Of our study sample, 0.10% of children suffered a pelvic fracture. The typical child with a pelvic fracture was a 12-15 year old female front row occupant of a passenger car involved in a struck side collision with intrusion. Analyses of the in-depth crash investigations determined that the developmental age of the child was the most important parameter associated with the type of pelvic fracture. Children 8-11 years experienced isolated pubic rami fractures, whereas children 12-15 years experienced multiple fractures of the pelvic ring. This distinct injury pattern is directly related to the ossification during puberty of the cartilage connecting the three bones of the pelvis. A secondary influence on the injury pattern was the geometrical and stiffness incompatibilities between impact partners that resulted in vehicle deformation and rotation of the vehicle seat. In the effort to improve the side impact protection of vehicles, attention should be paid to improved structural chassis design that minimizes the dynamic deformation of the seat.