Iatrogenic spinal arachnoid cysts are rare, but have been described as a complication of spinal injection and lumbar puncture procedures. The authors describe 2 cases of iatrogenic spinal arachnoid cyst formation that occurred after incidental durotomy during lumbar spine surgery. In both cases, postoperative MR imaging revealed compression of the cauda equina by an intradural arachnoid cyst. Intradural exploration and fenestration of the arachnoid cyst was accomplished in each case. This entity should be considered in the differential diagnosis of a patient experiencing symptoms of neurological compression after a lumbar surgery complicated by incidental durotomy.