A 10-year-old girl sustained a traumatic open-book pelvic fracture from a straddle injury in a jet ski accident. Plain films and computed tomography both demonstrated diastasis of the symphysis pubis and bilateral widening of the sacroiliac joints. The open-book fracture resulted from the patient's striking the steering column of the watercraft during a deceleration accident. The unusual cause of this injury is of clinical interest because with increasing popularity of personal watercraft and changes in the design of these vehicles, the incidence and prevalence of this type of injury may increase in the future.