Until recently, idiopathic anterior spinal cord herniation was thought to be vanishingly rare. It is an increasingly recognized, readily treatable, condition causing progressive thoracic myelopathy. We have operated on five cases and know of two other patients locally who are as yet undecided on surgery. We are aware of several other unpublished cases in the UK. The syndrome presents typically in middle age with a history of stepwise slowly progressive mid-thoracic anterior hemicord dysfunction characterized by hemianalgesia below the affected segment followed by asymmetric lower limb hypertonia, with initial sparing of posterior column sensation, that progresses through paraparesis to complete paraplegia. The radiological findings, although subtle, are equally typical and surgical repair is easily effected. With increasing clinical and radiological awareness, we believe the apparent incidence of this condition will increase. Idiopathic anterior spinal cord hernia is more common than currently appreciated, and should be positively considered and excluded in every case of progressive thoracic cord dysfunction.