The efficacy of topical aluminium chloride hexahydrate 20% W/W ethanol (ACH) in the treatment symptomatic palmar hyperhidrosis was studied in 12 patients. A half-sided control single blind (assessor blind) study was done. Patients applied ACH on one palm daily for 4 weeks. The response to treatment was measured objectively with an evaporimeter. There was significant fall of skin water vapor loss (SVL) on treated palms compared with untreated palms. The basal mean SVL of treated palms and untreated palms were 79.9 and 77.9 g water/m2/h, respectively (n.s.). The mean SVLs of treated vs. untreated palms at week 1, 2, 3, and 4 were 66.4 vs. 79.7 (p less than 0.05), 56.6 vs. 72.2 (p less than 0.001), 58.2 vs. 72.5 (p = 0.1), and 51.4 vs. 72.7 (p less than 0.001) g water/m2/h, respectively. The mean SVL of treated palms returned near basal rate within 1 week of stopping treatment. Four patients developed skin irritation from ACH; in three this disappeared after 1 week and they were able to continue with treatment; one withdrew from the study because of the severe irritation. All patients reported that the ACH reduced palmar sweating within 48 hours of application; its effect disappeared within 48 hours after stopping treatment. ACH appeared to be useful in rapid control of palmar hyperhydrosis.