11 cases are reported in which a detergent was inadvertently injd. into the spinal fluid with a spinal anesthetic. As a result, a clinical picture divided into 4 stages was initiated, some patients developing all, others showing recovery at the initial period. In some the clinical picture was that of a cauda equina syndrome. In others evidence of an added spinal cord syndrome developed, at times giving a level symptomatology. In the 3d group there were added cerebral manifestations including headache, apathy, confusion and cranial nerve signs, especially single or bilateral 7th and 6th. In the final or terminal stage there was continuation of the upward march of the pathologic process. Here increased intracranial pressure was the outstanding manifestation with evidence of progressive internal hydrocephalus. All 4 patients who progressed into the 4th stage died despite strenuous effort to halt the inevitable progress of the condition by ventriculostomy. These cases differ from all other "post-spinal anesthesia arachnoiditis" cases in that the symptoms came on within a few days after the intraspinal anesthesia. In half the cases a cerebral complication occurred. In 1 case a marked ventricular dilatation was visualized by air injn. through a ventriculostomy tube. Recovery in the milder cases began quickly even after a complete flaccid paralysis of the legs had occurred. The pathologic process consisted of a demyelinization of the spinal roots beginning with the cauda equina, with a marginal status spongiosus of the spinal cord itself. A hyperplasia of the pia resulted and was an ascending process, eventually involving the soft membranes of the brain and producing obstruction of outflow of cerebrospinal fluid with secondary hydrocephalus. The intensity and extent of the pathologic process probably depended on the concn. of the detergent soln. introduced into the spinal canal.