The major plasma metabolite of acitretin (trans-acitretin) is its 13-cis isomer, cis-acitretin. Interconversion of cis -acitretin to trans-acitretin was demonstrated in man following administration of a single oral dose of cis -acitretin. Plasma concentrations of Ro 13–7652 (cis-acitretin) and Ro 10–1670 (trans-acitretin) were much higher after cis -acitretin administration than after trans-acitretin administration. Surprisingly, these high concentrations were not associated with a clear therapeutic effect in dermatoses (e.g. psoriasis) which are usually responsive to oral retinoids. Interactions between the cis and trans isomers formed in vivo may explain the difference in therapeutic activity of each stereoisomer when administered ora