The clinical and radiological findings in six extradural and nine intradural arachnoid cysts are discussed in relation to previous reports. Only two cysts failed to opacify during positive contrast myelography; in both cases Myodil was used and in one of them contrast medium had entered the cyst on delayed films taken at 24 h. Pain was always improved and generally cured by operation, only two patients having some residual backache. There was permanent improvement of neurological function in only eight cases. The factors associated with poor permanent recovery after surgical treatment were: 1) very marked thinning of the spinal cord by the cyst, and 2) relatively longer duration of paresis--only one case had paresis for under 2 years (mean 4.8 years) compared with only two cases for over 1 year (mean 2 years) in those with good recovery.