When roots of the brachial plexus are avulsed, a cuff of dura-arachnoid is torn out of the spinal theca. With retraction and shrinkage of the severed stump, a cavity is created which fills with spinal fluid. It is gradually enclosed by re-formation of the ruptured arachnoid and dura, and a traumatic meningocele is formed. This can be readily filled with radiopaque media, and the presence and extent of root avulsion can be demonstrated by myelography.