Background: A common recommendation by many public health agencies is to reapply sunscreen every 2 to 3 hours. Is this recommendation effective in minimizing ultraviolet exposure of the skin during time in the sun? Objective: The purpose of this article is to determine how the time of sunscreen reapplication affects the solar ultraviolet exposure of the skin. Methods: A mathematical model was derived that took into account typical amounts of sunscreen application and sunscreen substantivity to determine how these factors, when combined with the time of sunscreen reapplication, influence the photoprotection provided by sunscreen during exposure for several hours around mid day in strong sunshine. Results: Using a sunscreen that is readily removed from the skin achieves little in the way of sun protection, no matter when it is reapplied. For sunscreens that bind moderately or well to skin, typical of modern waterproof or water-resistant products, the lowest skin exposure results from early reapplication into the sun exposure period, and not at 2 to 3 hours, after initial application. Typically reapplication of sunscreen at 20 minutes results in 60% to 85% of the ultraviolet exposure that would be received if sunscreen were reapplied at 2 hours. Conclusions: Advice given to sunscreen users should be to apply sunscreen liberally to exposed sites 15 to 30 minutes before going out into the sun, followed by reapplication of sunscreen to exposed sites 15 to 30 minutes after sun exposure begins. Further reapplication is necessary after vigorous activity that could remove sunscreen, such as swimming, toweling, or excessive sweating and rubbing. (J Am Acad Dermatol 2001;45:882-5.)