Retinoids (vitamin A and its derivatives) are potent substances for regulating the expression of many different genes within the body. The gene regulatory activities of retinoids are mediated primarily by the all-trans and 9-cis isomers of retinoic acid. Although 13-cis-retinoic acid (isotretinoin) does not have the potent gene regulatory activity of the other two isomers, it is an effective pharmacologic agent for treating a variety of dermatologic conditions. Because 13-cis-retinoic acid is also a naturally occurring retinoid that is present in the circulation, question is raised as to the biochemical mechanism(s) responsible for its pharmacologic efficacy. Some of this efficacy likely arises from the ability of 13-cis-retinoic acid to undergo isomerization to the significantly more active all-trans and 9-cis isomers; however, this does not account for all of the pharmacologic effects observed upon use of this retinoid. Some recent studies suggest that 13-cis-retinoic acid may act by inhibiting the actions of enzymes that are needed to metabolize steroids, while other recent studies indicate that 13-cis-retinoic acid acts through membrane receptors present on the surface of cells. At the present, it is not possible to rule out still other possible biochemical actions of 13-cis-retinoic acid in the body. It is clear, however, that if we are to fully understand the basis for the clinical efficacy of 13-cis-retinoic acid, a better understanding of such biochemical actions is needed. (J Am Acad Dermatol 2001;45:S129-35.)