Described nearly 70 years ago as the ‘punch drunk’ syndrome, dementia pugilistica has recently received increasing attention in both the medical and lay press. The association between the finding of cavum septi pellucidi and dementia in old boxers was first described by Ferguson and Mawdsley in 1965. Since then it has commonly been listed as one of the features of this condition. We review the significance and the epidemiology of cavum septi pellucidi and present an alternative explanation for the association between this developmental anomaly of the brain and dementia pugilistica.