Background: Aminolevulinic acid hydrochloride (ALA, Levulan) applied topically to actinic keratoses (AKs) leads to accumulation of the photosensitizer protoporphyrin IX, which, when activated by exposure to light, eradicates AKs. Objective: We examined the safety and efficacy of photodynamic therapy using topical 20% ALA in a solution formulation and varying blue light doses to treat multiple AKs on the face and scalp. Method: This is a multicenter, investigator-blinded, randomized, vehicle-controlled study. Results: Thirty-six patients with clinically typical AKs were treated with 20% ALA; 14 to 18 hours later, they were irradiated with a nonlaser fluorescent blue light source. With the optimal light dose of 10 J/cm², 88% of the AKs completely cleared 8 weeks after a single photodynamic treatment, compared with 6% after treatment with vehicle and light. Conclusion: Topical ALA PDT using a nonlaser, blue light source is an effective treatment for multiple AKs. (J Am Acad Dermatol 2001;45:96-104.)