In the present study, which aimed at opening a new trend of research on the biological consequences of IPL treatments, we analyzed 2 known markers of tissue photodamage, DNA thymine dimers and lipid peroxidation, hallmarks of UV-B12,13 and UV-A radiation,3,7,14,15 respectively, on human skin in vivo. If the absence of thymine dimer formation by IPL is comforting, which is not surprising because the action spectrum for pyrimidine dimer formation is centered on the absorption spectrum of pyrimidine bases (260 nm),12 the ability of IPL to induce a significant oxidative stress in the skin should induce IPL users to be careful when using IPL treatment in a recurrent manner. Oxidative stress is a well-documented consequence of UV-A irradiation,3,7,14,15 although the primary chromophores have not yet been identified. It is unlikely that the lipid peroxidation observed in this study was due to UV-A radiation because the IPL system was well filtered below 550 nm. On the other hand, Applegate and colleagues16 showed that infrared light (700-4000 nm) was not capable of inducing frank damage to DNA or oxidative stress proteins in fibroblasts in vitro, indicating that the small infrared component of our IPL system (800-950 nm) was probably not responsible for the lipid peroxidation shown in skin biopsy specimens. This oxidative stress was thus probably due to a moderate dose of visible light delivered through a very high fluence rate. This is a new observation, to our knowledge, and, as for UV-A, the primary chromophores remain to be identified. The histological examination of the biopsy specimens does not show any morphological alteration of both the dermis and the epidermis; moreover, no modulation of the proliferative state was observed, as shown by Ki67 staining. Although the biopsy specimens were taken only 2 hours after irradiation, which precludes the detection of delayed biological events such as apoptosis, epidermal hyperplasia, or leukocyte infiltration, this period would have been long enough to show acute reactions such as burning, swelling, or acute epidermal necrosis.