Background: Laser resurfacing has beneficial effects in the treatment of several skin problems such as acne and chicken pox scars. Recently pulsed erbium YAG laser has been used for skin resurfacing, but not enough is known about the scientific basis of such treatment. Objectiye : The purpose of the study was to assess the macroscopic and microscopie changes after irradiation with the pulsed Er-YAG laser. Methods : We did two tests on guinea pig skin, First, the laser was used to irradiate the skin of a guinea pig with the intensity of 500 mJ, 5 watt until pinpoint bleeding began. Skin biopsies were performed immediately, one, two and four weeks after the irradiation. Second, the laser was used to irradiate the skin of a guinea pig with the intensity of 300mJ(3watt), 500mJ(5watt), 700mJ(7watt) until pinpoint bleeding began, Skin biopsies were examined immediately, one, two and four weeks after the irradiation. Results : 1. Histological changes to the dermis over time after laser irradiation with 500 mJ, 5 watt are as follows: After one week, complete reepithelization and proliferation of the blood vessels within the dermis occured. A reaction of th dermis which consisted of newly formed collagen and was distinguisbed from the lower dermis was also examined. The reaction was rnost active after two weeks and lasted up to four weeks. 2. Histological changes to the dermis over time after laser irradiations with different intensities of 300 mJ(3watt), 500 mJ(5watt), and 700 mJ(7watt) are as follows : In all the cases of laser irradiation with different intensities, edema, neovascularization and fine collagen fiber in the upper dermis were observed. At higher intensities, the dermal reactions were stronger and were sustained for a longer time. Conclusion : Pulsed erbium YAG laser had effects on the epidermis and dermis. Therefore, pulsed erbium YAG laser can be used for skin resurfacing effectively and safely. (Kor J Dermatol 1997;35(4): 851-659)