Hyperhidrosis can be emotionally challenging and socially and professionally disruptive, and there are few effective treatments. This condition was successfully treated with botulinum toxin in two men who, since their early teens, had had excessive axillary sweating, requiring frequent shirt changes. They received bilateral axillary injections with 100 units of botulinum toxin type A, and within 5 days reported cessation of excessive sweating. Quantitative measurements before and 2 to 4 weeks after the injections demonstrated an average reduction of 71% and 76% (from 11.6 to 3.4 and from 2.5 to 0.6 mL/min m2) in axillary sweating during rest. A 96% reduction (from 42.9 to 1.7 mL/min m2) was seen in one patient during mental stress. No complications developed. This study quantitates the reduced axillary sweating achieved through chemodenervation with botulinum toxin A.