Sixteen patients with chronic leg ulcers were treated with 10% benzoyl peroxide gel for 6 weeks. At the end of the treatment, patch tests with 2% benzoyl peroxide in petrolatum and 10% in a gel showed that nine patients (56%) had become sensitized. This high sensitization rate is in good accord with the results obtained by experimental investigations using human maximization tests and repeated insult patch tests. A strong sensitizer such as benzoyl peroxide should not be used in the routine treatment of chronic leg ulcers, despite its good healing effect.