Two patients are described, of whom one suffered from progressive dementia, the other with a picture suggestive of Guillain-Barré syndrome. Both were found at necropsy to have small vessels throughout the body clogged with malignant cells with resultant cerebral infarcts. The source in one case was a 1-cm tumor in the thyroid, in the other a microscopic focus in the pancreas. It is suggested that most cases described as neoplastic angioendotheliosis involving the brain represent vascular dissemination of an unrecognized primary carcinoma rather than a miraculously widespread malignant endothelial transformation.