Idiopathic palmar and plantar hyperhidrosis is a relatively common disorder of eccrine sweat gland function. Treatment with glutaraldehyde of formaldehyde, although successful, may cause undesirable side effects. Methenamine is a polycyclic organic compound which release ammonia and formaldehyde at acid pH. Five per cent methenamine in a firm stick gel, applied daily to one palmar or plantar surface of 109 patients with hyperhidrosis, resulted in significantly less sweating after one month. No patients were sensitized to formaldehyde.