Sunscreen is used as a primary strategy to prevent sunburn and later skin cancer. However, sunscreen use has paradoxically been associated with the increasing incidence of skin cancer. One explanation for this puzzling observation is sunscreen failure (sunburn in the setting of sunscreen). Our purpose was to evaluate mechanisms of sunscreen failure in a sunscreen-using population. We carried out an epidemiologic comparison of sunburned and nonsunburned beachgoers who used sunscreen. We found that men were less likely to use sunscreen than women (χ² = 11.3, df = 1, P = .001), and when it was used, men were less likely to apply sunscreen to all sunlight-exposed skin (χ² = 18.4, df = 1, P = .0001). Swimmers who used sunscreen were significantly more likely to be sunburned compared with nonswimming sunscreen users (Fisher exact test, df = 1). Sunscreen may fail to prevent sunburn if it is washed off during swimming or if it is not applied to all exposed skin. Epidemiologic studies that link sunscreen use to skin cancer should evaluate whether sunburn occurred in this setting. (J Am Acad Dermatol 2001;44:781-4.)