The authors report six cases of so-called spinal subdural arachnoid cysts, emphasizing the clinical and myelographic findings which, if not properly evaluated, may be misleading diagnostically. The literature of so-called perineural cysts and of extradural arachnoid cysts is likewise reviewed. Their clinical, roentgenological, and pathological features are examined. A common pathogenesis for all these lesions is proposed. It implies disruption and secondary proliferation of the arachnoid membrane. Hence, the term arachnoid diverticulum is advanced to include all lesions communicating with the subarachnoid space. Their varying relations with the subarachnoid space depend on the sites of the primary abnormalities and on hydrodynamic factors. Numerous observations of associated arachnoid diverticula, either perineural, subdural, or extradural, further favour a common pathogenesis.