The purpose of this study was to investigate the prevalence of the anatomical abnormalities that can induce inadvertent dural puncture when performing caudal block. The anatomy of the lumbo-sacral area was evaluated using magnetic resonance imaging. In 2462 of the 2669 patients imaged, the dural sac terminal was located between the upper half of the 1st sacral vertebra and the lower half of the 2nd sacral vertebra. In 22 cases (0.8%), the dural sac terminal and the spinal canal were located at or below the 3rd sacral vertebra, and these were cases of simple anatomical variations. As regards pathologic conditions, there was one case of sacral meningocoele and 46 cases of sacral perineural cyst. In 21 cases (0.8%) out of the 46 perineural cyst cases, the cyst could be found at or below the 3rd sacral vertebra level. Inadvertent dural puncture may happen when performing caudal block in patients with such abnormal anatomy.