Four retinoids - retinol (vitamin A), tretinoin (retinoic acid), etretinate (the ethyl ester of trimethoxymethylphenyl retinoic acid), and isotretinoin (13-cis-retinoic acid, Accutane) - have been administered orally in humans for therapeutic purposes. A review of the available information on the clinical toxic effects of these substances indicated that, while they express similar spectra of toxicity, they also differ in the extent to which they affect various body systems. This suggests that differential efficacy of the retinoids may be related, in part, to the cutaneous sites of maximum activity.