We performed a prospective study of sweating in 40 patients with hemispheral brain infarction and 40 healthy controls to elucidate the clinical significance and prognostic value of sweating dysfunction in conjunction with brain infarction. We measured hidrosis quantitatively at six sites on each side of the body before and after a heating stimulus in the acute phase, at 1 month, and at 6 months after infarction. Excessive evaporation on the paretic side when compared with the nonparetic side was already found at baseline, but after the heating stimulus, this asymmetry reached statistical significance on the forehead, chest, forearm, and hand during the whole 6-month follow-up. Significant asymmetry in sweating occurred in 29 of the 40 patients (73%) in the acute phase of infarction, in 18 of 32 (56%) after 1 month, and in 28 of 33 (85%) after 6 months. Hyperhidrosis correlated with the severity of paresis and the presence of pyramidal tract signs. We conclude that sweating asymmetry seems to be an essential, long-lasting consequence of autonomic failure occurring in the majority of patients with hemispheral brain infarction.