The variations in clinical efficacy and toxicity may, in part, be due to differences in the pharmacokinetics of each retinoid. The rejuvenated interest in retinoids has ushered in a new era, not only in the treatment of dermatologie diseases, but in the treatment of preneoplastic and neoplastic disease as well. The proper retinoid compound will be selected according to its toxicity spectrum as well as the type of study in which it is to be used; these are as important as its biological activity. Most reports of retinoid toxicity have been in the dermatologie literature dealing with the treatment of acne and other dermatologie diseases. Acute toxicity may involve skin and adnexa, with diffuse hair loss in 10 to 75% of those using etretinate, acitretin, and to a lesser extent, isotretinoin. The exact mechanism of toxicity of vitamin A upon tissues is the increased lability of biological membranes and its surface “membranolytic” properties.