BACKGROUND Although there is increasing interest in the use of glycolic acid in the treatment of photoaged skin, to our knowledge, no controlled study has been done to assess the efficacy or the mode of this agent. OBJECTIVE The purpose of this study was to determine whether 50% glycolic acid can improve photoaged skin and to study the histological basis for this improvement. METHODS Forty-one volunteers were recruited into this double-blind vehicle-controlled study. Glycolic acid (50%) or vehicle was applied topically for 5 minutes to one side of the face, forearms, and hands, once weekly for 4 weeks. Punch biopsies were taken at pretherapy and at 5 weeks for histologic study. RESULTS Significant improvement noted included decrease in rough texture and fine wrinkling, fewer solar keratoses, and a slight lightening of solar lentigines. Histology showed thinning of the stratum corneum, granular layer enhancement, and epidermal thickening. Some specimens showed an increase in collagen thickness in the dermis. CONCLUSIONS The results of this study demonstrate that the application of 50% glycolic acid peels improves mild photoaging of the skin.