PURPOSE: To evaluate changes in the spinal canal in cases of L5 spondylolysis, as seen on sagittal MR images. MATERIALS AND METHODS: We retrospectively analysed the MR findings of 27 patients suffering from L5 spondylolysis without spondylolisthesis and compared them with 100 control subjects. Spondylolysis had been confirmed by conventional radiography. On midsagittal MR images, sagittal canal ratio (SCR) was defined as midsagittal canal diameter at L5 devided by that at L1. We analysed the frequency of posterior epidural fat deposition (posterior epidural fat between the posterior margin of the dural sac and the anterior cortical margin of the spinous process on the midsagittal line), and compared this with the frequency in 100 control subjects. RESULTS: Mean SCR value in 27 patients with L5 spondylolysis (1.22) was significantly greater than 100 control subjects (0.96, p<0.001). Mean SCR value in 17 patients with L5 spondylolysis and posterior epidural fat deposition (1.27) was significantly higher than in nine control subjects with posterior epidural fat deposition (0.97). Posterior epidural fat deposition was more frequently indentified in patients with L5 spondylolysis (63%) than in control subjects (9%). CONCLUSION: The possibility of L5 spondylolysis is suggested when on midsaggital MR imaging, the anteroposterior diameter of the L5 spinal canal is seen to be widened and posterior epidural fat deposition is noted.