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Summary From April 1st, 1955, to January 31st, 1956 a total of 60 cases of confirmed poliomyelitis of which 29 were non-paralytic, were under treatment. During the same period 15 cases were observed in which the diagnosis was acute encephalomyelitis. Motor weakness, rigidity, exaggerated reflexes without muscular wasting, sensory disturbances and normal cerebrospinal fluid suggested a disease other than poliomyelitis. This was confirmed by negative serological tests for all three types of poliomyelitis virus. Electromyography revealed absence of the denervation characteristic of poliomyelitis and the presence of long polyphasic units with reduced interference pattern characteristic of a myelopathic lesion. In one case a repeat examination after five months showed no improvement. In three cases with no evidence of neurological involvement the finding of characteristic electromyograms justified their inclusion in the series. The occurrence of reticulo-endothelial hyperplasia in some cases suggests that, as in poliomyelitis, there may be a viraemic phase after which the disease may abort. Proneness to fatigue and emotional instability were troublesome sequelae in at least 11 of the 15 cases, and in six cases motor weakness and/or sensory impairment have persisted. Five cases presenting behaviour changes were examined by electroencephalography. Although abnormalities were detected in all the cases, these were non-specific and may well be constitutional and not due to the condition under consideration. The pattern of the disease is similar to outbreaks which have occurred in different parts of the globe, notably Iceland, Australia, S. Africa and U.S.A. As far as can be ascertained this is a new disease in the sense that encephalitis lethargica (to which it has certain similarities) was a new disease appearing in epidemic form almost immediately after the first World War, presumably due to the widespread dispersal of susceptibles following demobilisation and migration: to a lesser degree poliomyelitis has similarly spread since 1945. Although there is little reason for complacency, the epidemiological scence can be viewed with justifiable confidence based on scientific methods of investigation and control.