The author describes a case and the pathological appearances found after death, and bases upon it some considerations regarding cerebral tumours. The patient for years had suffered from a monoplegia affecting the one arm, and was also subject to attacks of epilepsy. There was no headache, no vomiting, no disturbance of vision, and no weakening of the intellect. Twenty-five years after the beginning of these symptoms dementia supervened. At the autopsy, a tumour about the size of a small hen's egg was found in the middle of the right Rolandic region in correspondence with the posterior third of the frontal lobe. The tumour was an endothelioma, rich in cells probably of a sarcomatous nature, and without any nervous elements. The author points out the complete absence for twenty-five years of any mental symptoms is in complete accord with the theory of Bianchi, the unilateral nature of the lesion and the extraordinary slowness of its development giving time for compensation. An examination of the contents of the softening showed that the vessels remained normal; the nerve-fibres were very markedly altered, and the nerve-cells were reduced in number. The permanence of any cortical nerve elements was due to the slowly progressive interruption from the gradual compression and to the absence of any inflammatory process. The softening that surrounds tumours has been attributed by some to the compression of the small vessels, or to a superadded obliterative arteritis caused by syphilis or tubercle, the degeneration of the nervous elements following this. The author regards the extension of the softening to have been caused, not by the pressure of the tumour itself, but rather by pressure of fluid surrounding it. This produces in some cases an increase of tension, to which the white matter cannot offer sufficient resistive power, and hence the degeneration.