Thirty outpatients with limited (under 15% of skin area affected), symmetrical psoriasis of the plaque type were included in an open, right-left comparison study. Paraffin-based, 3% dithranol sticks (Ditrastick, Orion Pharmaceutica, Espoo, Finland) and similar, equimolar 4% butantrone sticks were used. Antipsoriatic activity, staining of the skin and clothes and irritation were the parameters followed up. An overall evaluation was made at the end of the trial. The antipsoriatic activity of dithranol and butantrone was almost equal at equimolar concentrations. Staining of the skin was significantly (at least p less than 0.05) milder with butantrone after 2 to 10 weeks treatment. Staining of clothes was less on the butantrone side for up to three months of use but was thereafter similar to that of dithranol. Irritation caused by butantrone reached its maximum only after 6 to 10 weeks and remained on average below the level of initial dithranol irritation. Six patients discontinued the treatment due to irritation by both preparations. Four patients tolerated better dithranol than butantrone. One patient could not use dithranol but cleared with butantrone. Thus, even if butantrone has an antipsoriatic efficacy almost equal to that of dithranol and is less staining than dithranol, it causes in some patients slowly increasing irritation with Koebner-like deterioration of the disease after some weeks of uneventful overnight use.