The reported prevalence of 5th ventricles based on air studies varies from 1-12% and ranges up to 60% as an autopsy finding. The prevalence of what is usually an incidental anomaly has not been determined by computed tomography (CT). 5th ventricles are however known to be more common in brain damaged boxers and with the introduction of compulsory CT scanning for certain boxers it is necessary to know what significance to attach to the finding of a cavum in these individuals. To ascertain the prevalence and morphology of 5th ventricles as detected by CT in the population, a thousand consecutive scans were analysed for the presence or absence of a 5th ventricle and other associated midline developmental abnormalities and correlations made with any pathology found. A 5th ventricle was present in 5.5% of the group and in most cases was less than 3 mm wide. An apparent association with other pathology was found only in patients under the age of 15. A 6th ventricle was found in 0.5% while a cavum velum interpositum was present in 9.5%. The isolated finding of a small 5th ventricle on the CT scan of a young active boxer almost certainly represents a persistent congenital anomaly of no significance.