A polycyclic aromatic compound having the formula ##STR1## and its salts have useful therapeutic and cosmetic action particularly in psoriasis. In the formula, X represents --CH.dbd.CH--, O or S, R.sub.1 represents hydrogen, branched alkyl having 3-15 carbon atoms, alkoxy having 1-6 carbon atoms or 1-adamantyl, R.sub.2 represents hydroxy, hydrogen, linear or branched alkyl having 1-15 carbon atoms or alkoxy having 1-6 carbon atoms, with the proviso that R.sub.1 and R.sub.2 are not simultaneously hydrogen, or R.sub.1 and R.sub.2 together with the adjacent carbon atoms of the naphthalene ring form a 5 or 6 chain ring optionally substituted by at least one lower alkyl radical, or interrupted by an oxygen atom, R.sub.3 represents --CH.sub.2 OH or --COR.sub.4, or --CH.sub.3 when R.sub.1 and R.sub.2 taken together form a 5 or 6 chain ring, R.sub.4 represents --OR.sub.5 or ##STR2## R.sub.5 represents hydrogen, alkyl having 1-20 carbon atoms, monohydroxyalkyl, polyhydroxyalkyl, aryl or aralkyl optionally substituted, or the residue of a sugar or the radical ##STR3## p is 1, 2 or 3, r' and r" represent hydrogen, lower alkyl, monohydroxyalkyl, polyhydroxyalkyl, aryl optionally substituted or the residue of an amino acid or the residue of an amino sugar, or taken together form a heterocycle.