Two cases of circumscribed cortical atrophy (Pick's disease) are described. The clinical differentiation of this affection from Alzheimer's disease and senile dementia is difficult. Histopathologically, there was atrophy of the left hemisphere in both instances; the temporal and the hippocampal convolutions showed the most extensive involvement. In the second case the atrophy also extended to the parietal convolutions. Practically all the cyto-architectural layers of the affected convolutions were distorted and the ganglion cells showed retrograde and other pathologic changes. In the second case the second and third cortical laminæ were more involved than the other layers. Of unusual interest in the first case were the atherosclerotic changes in the vessels of the left temporal lobe, the swelling and chromatolytic changes of the Purkinje cells of the cerebellum and of some of the nerve cells of the medulla oblongata.The various pathogenetic theories accounting for the production of this disorder are discussed.