As an investigative procedure, a hydro(5-FU) ointment containing 20% 5-fluorouracil (5-FU) was applied to the skin of patients with extensive actinic keratoses of the face and neck, for a period of four weeks. This resulted in a selective inflammation, erosion, and disappearance of the keratoses without significant alteration of the normal skin. Transitory adverse reactions included corneal and conjunctival irritations, phototoxic reactions, and erosion of the lower lip border. No evidence of systemic absorption was detected. Only preliminary follow-up observations are available, and no conclusion can be drawn as to the long-term results.