Background: The immune system plays a critical role in the development and pathogenesis of actinic keratosis (AK). Imiquimod has been shown to stimulate the cutaneous immune response and be effective for the treatment of nonmelanoma skin cancers. Objective: Two phase III, randomized, double-blind, vehicle-controlled studies evaluated the efficacy of imiquimod 5% cream compared with vehicle in the treatment of AK lesions on the face and balding scalp. Methods: A total of 436 participants at 24 centers in the United States and Canada were randomized to either imiquimod 5% or vehicle cream. Study cream was applied one time per day, 2 days per week for 16 weeks. Clearance of AK lesions was clinically assessed at an 8-week posttreatment visit. Results: The complete clearance rate was 45.1% for the imiquimod group and 3.2% for the vehicle group. The difference in complete clearance rates (imiquimod minus vehicle) was 41.9% with a 95% confidence interval of 34.9% to 49%. The partial (≥75%) clearance rate was 59.1% for the imiquimod group and 11.8% for the vehicle group. The difference in partial clearance rates (imiquimod minus vehicle) was 47.3% with a 95% confidence interval of 39.5% to 55.1%. The median percent reduction in AK lesions was 83.3% for the imiquimod group and 0% for the vehicle group. Local skin reactions were common. Severe erythema was reported by 17.7% of participants who received imiquimod and 2.3% of participants who received vehicle. Overall, imiquimod was very well tolerated. Conclusion: Imiquimod 5% cream used 2 times per week for 16 weeks is an effective and well-tolerated treatment for AK.