Retinoid transfer into breast milk was studied in a psoriatric woman receiving oral acitretin at a dosage of 40 mg once daily. Concentrations of the parent compound and its main metabolite, 13-cis acitretin, were measured in serum and mature milk during the initial nine days of therapy, using reverse-phase high performance liquid chromatography. At steady-state, trace amounts of the drug and metabolite (30-40 ng/ml) appeared in breast milk corresponding to a milk/serum concentration ratio of about 0.18. Acitretin was almost exclusively distributed in the fatty layers of the milk. Although the estimated amount of the drug consumed by a suckling infant would correspond to only 1.5% of the maternal dose, the toxic potential of acitretin justifies its avoidance in breast-feeding women.