The effects of topically applied sodium lauryl sulfate were studied in vivo, in man, through noninvasive methods and compared with those obtained in vitro, using human stratum corneum as a model. The results show that the very marked increase of transepidermal water loss obtained in vivo cannot be related to any removal of epidermal lipids. The most likely explanation of our findings lies in the hyperhydration of stratum corneum consecutive to the inflammation process and a possible disorganization of the lipid bilayers.