The brain stem is the most important autonomic processing center, but very little attention has been given to clinical manifestations of autonomic failure in brain stem stroke. Our purpose was to evaluate the prevalence, characteristics, and prognostic significance of sweating dysfunction in brain stem infarctions. We carried out a prospective study using quantitative evaporimetry to investigate spontaneous and heat-stimulated sweating in 18 healthy control subjects and 18 patients with ischemic brain stem stroke in the acute phase and at 1 and 6 months after infarction. The sweating response induced by a heating stimulus was significantly lower on the ipsilateral side to the infarction than on the contralateral side. Constant ipsilateral hypohidrosis was established in 83% of the patients in the acute phase, in 100% at 1 month, and in 76% at 6 months after infarction. No differences of sweating response were found between medullary and pontine infarcts. Hypohidrosis throughout the whole ipsilateral side of the body, a long-lasting phenomenon that has not previously been described, is an essential feature of autonomic failure in brain stem infarction.