Solar radiation causes immunosuppression that contributes to skin cancer growth. Photoprotective strategies initially focused on the more erythemogenic ultraviolet B. More recently, the relationship of ultraviolet A and skin cancer has received increased attention. We hypothesized that if ultraviolet A contributes significantly to human ultraviolet-induced immune suppression, then increased ultraviolet A filtration by a sunscreen would better protect the immune system during ultraviolet exposure. Two hundred and eleven volunteers were randomized into study groups and received solar-simulated radiation, ranging from 0 to 2 minimum erythema dose, on gluteal skin, with or without sunscreen, 48 h prior to sensitization with dinitrochlorobenzene. Contact hypersensitivity response was evaluated by measuring the increase in skin fold thickness of five graded dinitrochlorobenzene challenge sites on the arm, 2 wk after sensitization. Clinical scoring using the North American Contact Dermatitis Group method was also performed. Solar-simulated radiation dose-response curves were generated and immune protection factor was calculated using a nonlinear regression model. Significance of immune protection between study groups was determined with the Mann-Whitney-Wilcoxon exact test. The sunscreen with high ultraviolet A absorption (ultraviolet A protection factor of 10, based on the in vivo persistent pigment darkening method) and a labeled sun protection factor of 15 demonstrated better immune protection than the product that had a low ultraviolet A absorption (ultraviolet A protection factor of 2) and a labeled sun protection factor of 15. Nonlinear regression analysis based on skin fold thickness increase revealed that the high ultraviolet A protection factor sunscreen had an immune protection factor of 50, more than three times its sun protection factor, whereas the low ultraviolet A protection factor sunscreen had an immune protection factor of 15, which was equal to its labeled sun protection factor. This study demonstrates that ultraviolet A contributes greatly to human immune suppression and that a broad-spectrum sunscreen with high ultraviolet A filtering capacity results in immune protection that exceeds erythema protection. These results show that high ultraviolet A protection is required to protect against ultraviolet-induced damage to cutaneous immunity.