Nabumetone, a new nonsteroidal antirheumatic drug (NSAD), is chemically a (6-methoxy-2-naphthyl) butan-2-one. The drug was investigated with regard to its effectiveness and tolerability in a 3-wk open multicenter study involving 40 patients with osteoarthrosis and 45 with rheumatoid arthritis. Eleven patients with arthrosis of the hip and 8 suffering from rheumatoid arthritis were continuously treated for 6-12 mo. Evaluation showed a good result in 85% of the patients with osteoarthrosis and in 80% of the rheumatoid patients during 3 wk of therapy. Side effects were comparable with those of other NSAD. Changes in laboratory findings were not clinically significant. During long-term therapy good tolerability and persisting efficacy were shown.