An improved method suitable for the determination of 8-methoxypsoralen in the range 50-1500 ng/ml in the plasma of psoriatic patients undergoing PUVA (psoralens and long-wave ultraviolet light) therapy is proposed. A 5-ml aliquot of plasma containing sodium citrate as anticoagulant was centrifuged, griseofulvin was added as internal standard and the sample was denatured with acetonitrile. The supernatant was applied to C18 cartridges and 8-methoxypsoralen was eluted with methanol. The evaporated eluate was reconstituted in the mobile phase for high-performance liquid chromatography (HPLC) and applied to the HPLC column: mobile phase, acetonitrile-0.01 M phosphoric acid (34:66); flow-rate, 1 ml/min; temperature, 40 degrees C; column, Spherisorb 5 ODS, 100 mm x 4.6 mm I.D., 5 microns particle size; UV detection at 248 nm; detection limit, 15 ng/ml of plasma.