Currently, no documentation correlates histological changes with clinical signs of depth of the trichloroacetic acid peel. Obagi identified clinical signs of depth of injury following topical trichloroacetic acid application, employing prepeel conditioning and a method for slowing trichloroacetic acid action. A three-part study of 20 patients was undertaken to determine whether Obagi's visual and palpatory signs of depth correlated histologically with depth of peel. Also analyzed were physiological mechanisms associated with these signs. Patients were pretreated and biopsy specimens were harvested before and after modified trichloroacetic acid peeling. The results largely confirmed the validity of Obagi's observations regarding the method of trichloroacetic acid peel described. These clinical signs are verified by histology and correlated with some findings by electron microscopy. Differentiation of papillary from upper reticular dermal penetration is particularly useful. Physiological explanations for the phenomena observed are proposed. The specificity and safety of peels may be improved with these criteria.