The percutaneous penetration and the metabolism of benzoyl peroxide (BPO) were assessed in vitro on human skin and in vivo on 5 patients with leg ulcers. The BPO in vitro absorbed by the skin was converted to benzoic acid preferably in the dermis. The portion penetrated through the skin was benzoic acid only. Also in patients treated with BPO, no BPO could be detected in the serum. These findings show that BPO as such is absorbed by the skin, but is systemically absorbed only after the metabolisation to benzoic acid. Therefore a systemic-toxic effect in local therapy with BPO can be excluded.