A lumbar hernia usually involves protrusion of extraperitoneal fat or bowel through an area of weakness in the posterolateral abdominal wall bounded superiorly by the 12th rib, inferiorly by the iliac crest, posteriorly by the erector spinae muscle, and anteriorly by the posterior border of the external oblique muscle. Most are due to an acquired nontraumatic or congenital cause. Acute blunt abdominal trauma is a rare cause of lumbar hernia; to our knowledge, the CT diagnosis of this variety has not been reported. Since 1985, approximately 850 patients have undergone emergent abdominal CT for evaluation of acute abdominal trauma at our hospital; in seven of these patients, a traumatic lumbar hernia was diagnosed prospectively. In three patients, CT showed a flank hematoma with herniation of bowel through the lumbar triangle. CT showed pelvic fractures in three other patients, accompanied by herniation of bowel in one patient, herniation of extraperitoneal fat in another, and herniation of extraperitoneal fat and blood in the third. One patient had both a flank hematoma and a pelvic fracture with herniation of bowel. Acute traumatic lumbar hernia is a rare but significant abnormality that should be considered in patients with blunt abdominal trauma, especially in those with large flank hematomas and pelvic fractures. The hernia contents, associated injuries, and disrupted muscle layers are all well demonstrated on CT.