Background: Phototherapy has been studied as an alternative option for the treatment of acne. Propionibacterium (P.) acnes produces endogenous porphyrins as a by-product of metabolism. Light of particular wavelengths activates the porphyrins and induces a photodynamic reaction which subsequently kills the P. acnes. Objective: The purpose of this study was to evaluate and analyze the photoinactivation efficacy of different wavelengths against P. acnes in vitro. Methods: Using an LED lamps to emit light of variable wavelengths (370 nm, 385 nm, 395 nm, 405 nm, 470 nm, 505 nm, 590 nm, 630 nm and 880 nm), we examined the change of P. acnes colonies on agar plates after irradiation for 1, 3, 6 and 12 hours. Further investigations were also performed by irradiation on liquid medium, once, twice or three times consecutively, with an interval of 24 hours. Results: Light irradiation of wavelengths 385 nm, 395 nm, 405 nm and 470 nm, was found to decrease the number of P. acnes colonies on agar plates by about 50%. In liquid medium, we observed the photoinactivation effect of light irradiation on P. acnes with the wavelengths 370 nm, 385 nm, 395 nm, 405 nm and 470 nm. However, we observed no photoinactivation effect on P. acnes with the wavelengths 505 nm, 590 nm, 630 nm or 880 nm. Conclusion: By in vitro testing, we have demonstrated that light irradiation of wavelengths up to a maximum of 470 nm are effective in decreasing P. acnes and therefore may be a good alternative treatment modality for acne. (Korean J Dermatol 2006;44(11):1332~1338)