Posterior apophyseal ring fracture (PARF) of the lumbar spine is an uncommon injury thought usually to occur in adolescence. We analysed CT findings in 20 cases (19 patients), whose age ranged from 16 to 45 years (average 30 years). The PARF could be divided into two distinct groups on the basis of the shape and location of the fracture and the defect of the vertebral rim. In group 1 (2 cases), PARF involved the central aspect of the inferior vertebral rim and bone fragments were large and broad-based. In group 2, PARF were located at the posterolateral margin of the superior vertebral rim and bone fragments were small and focal. The great majority of previously reported cases belonged to our group 1. A pathogenesis for each group is suggested.