Seven cases of Huntington's disease (HD) showing unusual neostriatal findings are reported. In these patients, the neostriatum contained scattered islets of relatively intact parenchyma. We attempted to determine whether these cases might represent a clinico-pathological entity, and also sought to acquire a better understanding of the neostriatal degenerative changes in HD. Cell counts showed that the number of neurons in the islets was the same as in the normal neostriatum, but there was an increased number of astrocytes. The nature of these islets is poorly understood. Analysis of their topography and of the morphometric data indicates that they probably do not represent preserved striosomes. In HD patients with preserved neostriatal islets, the symptoms appear earlier, and the course of the disease is more rapid than in HD patients with the usual neostriatal lesions. The exceptional neuropathological phenotype of our seven HD cases probably indicates a different genotype.