Background: Acne is a multifactorial disease. Particularly, Propionibacterium acne acts on triglycerides and releases cytokines, which trigger inflammatory reactions and alter the infundibular keratinization status. Recently, visible light, especially blue light, has been shown to activate the endogenous porphyrins of P. acne, which results in a photodynamic reaction that destroys the bacteria. However, the effect of blue light on inflamed lesion, inflammatory papules, pustules, are largely unknown. Objective: We investigated the bactericidal effect of blue light on P. acnes as well as the inhibitory effect on the expression of inflammatory cytokines in HaCaT cells. Methods: We performed the colony count assay in blue light irradiated-culture media of P. acnes and RT-PCR analysis in blue light irradiated-HaCaT immortalized keratinocytes. Results: A 7 minute irradiation with blue light was bactericidal to P. acnes. It decreased the expression of tumor necrosis factor (TNF)-α and toll-like receptor (TLR) 2 in HaCaT cells. Expression of TNF-α-induced TNF-α and TLR 2 were down-regulated by blue light in HaCaT cells. Conclusion: Blue light is a potential anti-acne treatment. (Korean J Dermatol 2009;47(11):1213~1219)