The hypothalamus was systematically examined in 30 patients with pathologically documented idiopathic parkinsonism. Using Lewy body formation as a marker for nerve cell degeneration, we observed abnormalities in every hypothalamus examined. Of the thirteen hypothalamic nuclei that could be individually identified, none were exempt from Lewy body degeneration. The tuberomamillary nucleus and the lateral and posterior hypothalamic nuclei demonstrated the highest average Lewy body counts (16, 9, and 3, respectively); they were also the most frequently involved nuclei. These findings may aid in interpretation of the autonomic and endocrine abnormalities in patients with Parkinson disease.