A 21-year-old man presented with a 6-year history of episodic localized hyperhidrosis on the back of his hands. Results of the neurologic examination were normal. Profuse sweating of the affected skin was elicited by exercise, raising core temperature, direct heat, and intradermal injections of cholinergic drugs. Examination of the affected skin by electron microscopy and light microscopy with special stains and enzyme methods revealed prominent hyperplasia of otherwise normal eccrine sweat glands and ducts and increased surrounding stromal hyaluronic acid.