The effect of topically applied dithranol on the capillary resistance (CR) was studied both in healthy volunteers and in clinically unaffected skin of psoriatic patients. The CR of the psoriatic patients was significantly lower than that of the healthy persons. O.10% and 0.25% dithranol ointment induced a significant decrease of CR in the skin of both groups. This decrease was prevented or at least reduced, if the patients had previously been treated with either the antihistamine clemastine or the cyclooxygenase-inhibitor indomethacin. The effect of dithranol on the CR was also moderated, if 5% coal tar had been added to the dithranol preparation.