One hundred and thirty-five patients with lysis in the fifth (lowest) lumbar vertebra and 24 patients with L4 lysis were compared retrospectively with respect to radiographic variables and the occurrence of trauma before symptom onset. Hypoplasia occurred only in the L5 lysis group, whereas trauma as a symptom-precipitating factor was frequent in the L4 group. These observations suggest the possibility of different etiologies: trauma may contribute to lysis in a normal L4 vertebra, and inherited hypoplasia of the lumbosacral junction may lead to L5 lysis.