A number of serum components, whose concentrations or gene expression have been shown to be modulated by all-trans retinoic acid in vitro, were monitored in patients before and during treatment with Roaccutane™ (13-c/s retinoic acid, 40-60 mg/day) for severe acne. The 13-cis retinoic acid concentration in serum rose from 5.25 ± 1.09 to 593 ± 65 nmol/1 (mean ± SD) 24 h after the latest dose. The concentration of all-trans retinoic acid in serum under Roaccutane™ treatment was measured in model experiments and shown to be 10-20 nmol/1 i.e., 2-4 times the basal levels (4.65 ± 0.85 nmol/1) when the 13-cis retinoic acid concentration was 370-980 nmol/1. The concentrations of creatine kinase-MB, apolipoprotein B, total cholesterol and LDL cholesterol increased significantly while the other measured serum components, including lipoprotein lipase activity, were unaffected by Roaccutane™ treatment.