BACKGROUND: Intense pulsed light (IPL) generates high-intensity short flashes of visible light and has been used for about 10 years to improve dermatological conditions such as telangiectasia, pigmented lesions, and skin aging. Although these systems deliver a moderate dose (10-30 J/cm(2)) of visible light, this dose is delivered during a short pulse (2-5 milliseconds), which implies a very high fluence rate (approximately 4000 W/cm(2)). For this reason, we speculated whether the Bunsen-Roscoe law of reciprocity could still be valid in these conditions. OBSERVATIONS: Nine healthy volunteers were exposed to IPL or UV-A or simulated solar UV radiation, and then thymine dimer and lipid peroxide concentrations were determined in skin biopsy specimens of the exposed sites. Only exposure to solar UV radiation (7-J/cm(2) UV-A + 80-mJ/cm(2) UV-B) produced measurable amounts of thymine dimers in DNA from skin biopsy specimens, whereas UV-A radiation (40 J/cm(2)) and IPL (9 J/cm(2)) induced 3-fold and 6-fold increases of cutaneous lipid peroxides, respectively. CONCLUSIONS: These preliminary results indicate that IPL, although filtered for wavelengths shorter than 500 nm, can generate oxidative stress, a typical hallmark of UV-A, but does not induce thymine dimers. This emphasizes the need for long-term studies involving IPL before using this technique in a recurrent manner.