In a controlled double blind study the therapeutic efficacy and tolerability of [1-(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetoxyl]-acetic acid (acemetacin, TV 1322, Rantudil¿) and indometacin were tested in 40 patients (15 male, 25 female) suffering from psoriatic arthritis in which, in a period of three weeks, the patients received 180 mg acemetacin or 150 mg indometacin daily in form of three equally divided doses. In comparison with the initial values the acemetacin group in all three parameters tested, i.e., "pain, "limitation of function", "other signs of inflammation", showed a significant trend of improvement, while in the indometacin group only pain and inflammation could be improved significantly. With regard to the therapeutic ratios no difference could be established statistically between both groups. In the indometacin group side effects were observed nearly 4 times more frequently than in the acemetacin group; this difference is statistically significant (p < 0.1%). During the therapy with indometacin two withdrawals were necessary (1 because of inefficiency and 1 because of gastric complaints). In the acemetacin group there were no withdrawals. In 55% of all cases the therapeutic efficacy of acemetacin was judged as "good", in contrast to only 30% of the cases in the indometacin group. Acemetacin with regard to efficacy and tolerability seems to be clearly superior to indometacin.