An unstable displaced midline longitudinal (vertical) fracture of the sacrum, apparently not previously reported, is described. Pitfalls in treatment are elucidated. It is recommended that Pennal's classification be used to categorize all pelvic fractures in order to ensure early and adequate treatment and minimize the occurrence of disabling sequelae. In the patient reported, complications were bladder prolapse, pelvic instability, and sciatic nerve injury.