Fifty-two patients with acute and subacute transverse myelopathy (TM) were evaluated at the Massachusetts General Hospital between 1955 and 1975 and followed for 1 to 23 years (average, 5). Nineteen had symptoms of a recent acute infectious illness, 3 had cancer, and 1 had undergone a recent operation. There were four types of initial symptom. Twenty-four patients had paresthesias at the onset of the illness, 18 had pain, usually interscapular, 7 had leg weakness, and 3 had urine retention. Prognosis depended on the nature of the onset of TM. An acute catastrophic onset was generally associated with back pain and led to a poor outcome in 7 and a good outcome in only 1 of 11 patients. A subacute progressive onset over several days to four weeks, generally with ascending paresthesias or leg weakness, was associated with a good outcome in 15 and fair outcome in 17 of 37 patients. Preceding febrile illness, treatment with corticosteroids, and the nature of CSF abnormalities had no effect on outcome. Multiple sclerosis evolved in 7 patients during the follow-up period. Because of the frequency with which mass lesions were missed, the need for myelography in the diagnosis of TM is emphasized. The distinguishing clinical characteristics of TM provide guidelines for diagnosis and prognosis.