Acute spondylolytic spondylolisthesis was diagnosed after major trauma in five patients. The level of injury was between the third and fourth lumbar vertebrae in one patient and between the fifth lumbar and first sacral vertebrae in four. The initial spondylolisthesis was grade I in four patients and grade III in one. Four of the patients were initially managed non-operatively. The deformity did not progress in a five-year-old boy with grade-I spondylolisthesis who had been managed with immobilization in a body cast. The deformity progressed in two of the adolescents who had been managed non-operatively; the progression was from grade I to grade III in one of these patients and from grade III to grade V (spondyloptosis) in the other, in whom a cauda equina syndrome also developed. The latter patient was subsequently managed with posterior reduction and arthrodesis followed by an anterior arthrodesis, and the neurological deficits resolved. The deformity also progressed, from grade I to grade II over three years, in a fifty-seven-year-old woman who had been managed non-operatively. One patient who had a grade-I deformity was managed with immediate operative stabilization followed by immobilization in a thoracolumbosacral orthosis; the deformity did not progress. Although minor or repetitive trauma is often associated with spondylolysis, high-energy trauma may produce a more severe form of spondylolysis with spondylolisthesis. These deformities are more unstable, with instability similar to that of a fracture-dislocation, and they have a greater propensity to progress than the usual form of spondylolytic spondylolisthesis.(ABSTRACT TRUNCATED AT 250 WORDS)