Report on general data concerning the incidence of acute or sub-acute myelitis associated to radicular and/or leptomeningeal involvement (MMR) among patients observed in the neurologic ward of Hospital das Clínicas (São Paulo) over a 6 year period (1959-1965). Thirty one cases were registered: in all of them clinical and/or laboratory data did not show any possible etiological condition, as well did not make possible to exclude viral etiology. For comparative purposes data concerning 45 patients with Guillain-Barré syndrome (PRN) observed in the same period were studied. In MMR incidence no difference was related as to the sex of patients. There were proportionally more Negro people among the series of MMR patients than among PRN patients. About the half of the MMR cases were no more than 30 years old when the disease occurred. Most of the MMR cases studied (81%) occurred in the hot months of the year (from September to April). Remission of symptomatology occurred in about half of the MMR cases studied. In most of them it was partial and recovery of the patients incomplete.