The striatum, pallidum and subthalamic nucleus were studied by combined morphometric methods in serial sections of 13 brains of normal adults and of 15 patients with choreatic diseases. In addition the volume of the hemispheres and of the cortex were measured. All data obtained were corrected by the shrinkage factor to represent fresh brain values. In Huntington's chorea the pallidum was more severely affected than is commonly appreciated. The average volume reduction was of the same degree (lateral-57%, medial-50%) as that of the striatum (-56%). The absolute number of nerve cells of the pallidum decreased in both segments by about 40%. The reduction of the volume and of the number of nerve cells was not reduced in the three subcortical nuclei studied. For the first time it has been shown that there is no increase in the absolute number of glial cells in the striatum. The increased numerical density of glial cells is caused by shrinkage. The loss of nerve cells of the pallidum and subthalamic nucleus is caused mainly by a primary process. Huntington's chorea is a multifocal process. Morphometric data do not suggest that subchorea is a variant of Huntington's chorea. Chorea minor is regarded as a multifocal process with varying affliction of the striatum, pallidum and subthalamic nucleus. An increase in the number of glial cells and, as a rule, a moderate loss of nerve cells were found in this disease.