N, N', N"-Triethylenethiophosphoramide [Thio-TEPA (NSC 6396)] is the third drug to be evaluated for the treatment of meningeal neoplasia. Eleven patients with meningeal leukemia, lymphoma, or ependymoma were treated with intrathecal thio-TEPA in doses from 1 to 10 mg/m2 of body surface area. There was no hematologic toxicity definitely attributable to thio-TEPA, and neurologic toxcity was limited to mild transient paresthesias of the lower extremities during lumbar sac injection in three patients. Two of those experiencing such paresthesias received concentrated drug solutions to decrease the large injected volumes associated with the higher dosages of thio-TEPA. Three patients achieved complete meningeal remission, and five others had a partial response to therapy.