Two cases of congenital extradural cyst are reported. No similar cases occurring in the same family have ever been recorded in the literature. Myelographic examination in the conventional prone position revealed an extradural block of no specific nature. It was only in the supine position, that the contrast medium could flow into the cyst from the subarachnoid space, thus making a definitive diagnosis possible. Myelographic blocks at 3 different levels were almost identical in the 2 cases. The formation of the 3 myelographic blocks is explained. The literature is briefly reviewed and the syndrome is described.