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Background Ultraviolet (UV) radiation can profoundly suppress the cutaneous immune system, thus enhancing carcinogenesis. Agents that prevent UV-induced immunosuppression may thus reduce skin cancer. Nicotinamide (vitamin B3) prevents UV-induced immunosuppression and carcinogenesis in mice, and solar-simulated (ss) UV-induced immunosuppression in humans. Its effectiveness against different UV wavebands and mechanism of action is as yet unknown. Objectives To determine the effects and mechanisms of topical nicotinamide on UV-induced suppression of delayed type hypersensitivity (DTH) responses in humans. Methods Healthy Mantoux-positive volunteers in four randomised, double-blinded studies were irradiated with solar-simulated (ss)UV (UVB + UVA) or narrowband UVB (300 nm) or UVA (385 nm). Topical nicotinamide (0·2% or 5%) or its vehicle were applied immediately after each irradiation. Mantoux testing was performed at irradiated sites and adjacent unirradiated control sites 48 h after the first irradiation and measured 72 h later. Immunosuppression was calculated as the difference in Mantoux-induced erythema and induration at test sites compared to control sites. Human keratinocyte cell cultures, with and without ssUV and nicotinamide, were used for quantitative real-time reverse transcriptase-polymerase chain reaction assessment of TP53 and enzymes that regulate oxidative phosphorylation. Results Nicotinamide cooperated with ssUV to increase enzymes involved in cellular energy metabolism and p53, and significantly protected against immunosuppression caused by UVB, longwave UVA and single and repeated ssUV exposures. Conclusions Longwave UVA, which is poorly filtered by most sunscreens, was highly immune suppressive even at doses equivalent to 20 min of sun exposure. Nicotinamide, which protected against both UVB and UVA, is a promising agent for skin cancer prevention. [ABSTRACT FROM AUTHOR]