Abstract: A pharmaceutical composition for use in the treatment of psoriasis comprises 4-aminosalicylic acid (4-ASA) or 5-aminosalicylic acid (5-ASA) or a functional derivative thereof, said pharmaceutical composition being in a form suitable for topical administration. Furthermore, 4- or 5-ASA or a functional derivative thereof are used for the manufacture of pharmaceutical compositions for treating psoriasis, atopic dermatitis, allergic dermatitis, contact dermatitis, seborrhoic dermatitis, or acne diseases. The derivatives have the formulae: ##STR1## where W is COOX, wherein X is H, Li, Na, K, Mg.sub.0.5, Ca.sub.0.5, Zn.sub.0.5, Al.sub.0.33, Fe(II).sub.0.5, Fe(III).sub.0.33, NH.sub.4, NH.sub.3 R.sup.1, NH.sub.2 R.sup.1.sub.2, NHR.sup.1.sub.3, NR.sup.1.sub.4, or R.sup.1, where R.sup.1 is substituted or unsubstituted C.sub.1-6 -alkyl, aryl-C.sub.1-4 -alkyl, or heteroaryl-C.sub.1-4 -alkyl; or COX, where X is NR.sup.1 R.sup.1', where R.sup.1' has the same meaning as R.sup.1 defined above and R.sup.1 and R.sup.