Irritation and staining by dithranol and some related compounds was studied with the chamber-testing technique in 62 psoriasis patients. Exposure of the mid-back skin to 0.05 and 0.5% dithranol for 24 h and reading of the reactions (erythema, brown staining, edema) on the 2nd day after application is a suitable regimen for testing tolerance. Contact thermography was used when the brown stain hampered the estimation of erythema. Low concentrations of 10 acetyldithranol elicited weaker reactions than dithranol but at the 0.5% level the acetyl-derivative tended to give stronger reactions than the parent compound. Dithranol triacetate (10%) caused slight erythematous reactions only occasionally, but 10,10''-bis(formylethyl) dithranol and dithranol anthraquinone elicited no reactions at all. The mechanisms of the staining and inflammatory reactions and possibilities of their reduction are discussed. The use of the chamber-testing technique is recommended for screening of new less irritating and staining hydroxyanthrones for clinical trial.