OBJECTIVE: Post-void residual (PVR) scans of less than 200 ml are increasingly being used to rule out the likelihood of cauda equina syndrome (CES) and to delay emergency MRI scanning in suspected cases. This study was done to review a series of 50 MRI confirmed cases of CES and to test the hypothesis that a PVR of less than 200 ml was unlikely to be present. METHODS: Fifty consecutive medicolegal cases involving CES were audited. Records were reviewed to see if PVR scans were done. MRI scans were reviewed, clinical and radiological diagnosis reviewed, and treatment recorded. RESULTS: Out of 50 CES cases, 26 had had PVR scans. In 14/26 (54%) the PVR scan was ≤ 200 ml. In one case, the CES diagnosis was in question leaving 13/26 (50%) cases where there was a clear clinical and MRI diagnosis of CES despite the PVR being ≤ 200 ml. All 13 were classified as incomplete cauda equina syndrome (CESI) and all proceeded to emergency decompression. CONCLUSIONS: This study is the first in the literature to demonstrate that there is a significant group of CES patients who require emergency decompression but have PVRs ≤ 200 ml. The results demonstrate the existence of a significant group of CESI patients whose bladder function may be deteriorating, but they have not yet reached the point where the PVR is over 200 ml. Given the accepted understanding that CESI is best treated with emergency decompression, such patients are likely to have worse outcomes if MRI scanning and therefore surgery is delayed. We recommend the following: PVR is recommended as an assessment tool in suspected CES. A PVR of ≤ 200 reduces the likelihood of having CES but does not exclude it; clinical suspicion of CES should always lead to an MRI scan. Further investigation of PVR as a prognostic tool is recommended.