The sap of the plant Euphorbia peplus is a traditional remedy for skin conditions, including actinic keratosis. The active constituent of the sap is ingenol mebutate (ingenol-3-angelate), formerly known as PEP005. This randomized, double-blind, vehicle-controlled, phase IIa study investigated the safety (and secondarily the efficacy) of two applications of ingenol mebutate gel in 58 patients with biopsy-confirmed actinic keratosis. Five preselected lesions were treated with ingenol mebutate gel, 0.0025%, 0.01% or 0.05%, or vehicle gel, on days 1 and 2 (Arm A) or days 1 and 8 (Arm B). There were no significant differences in tolerability or efficacy between Arms A and B. Treatment was well tolerated. The most common local skin responses were dose-related erythema, flaking/scaling/dryness and scabbing/crusting. Efficacy was greatest with ingenol mebutate gel, 0.05%, which resulted in complete clinical clearance of 71% of treated lesions (P < 0.0001 vs vehicle gel). In addition, 67% of patients treated with ingenol mebutate gel, 0.05% had clinical clearance of at least four of five treated lesions (P = 0.0185 vs vehicle gel). Ingenol mebutate gel is being developed as a short-course topical therapy for actinic keratosis and non-melanoma skin cancer.