The aim of the present study was to compare the efficacy and tolerability of a new form of nonsteroidal antiinflammatory drug (NSAID), acematacin (Acemet), with indomethacin, in patients suffering from osteoarthritis (OA). Patients diagnosed with OA were included in a double-blind, randomized, controlled parallel group study A total of 42 patients were randomized to either acemetacin 60 mg t.i.d. or indomethacin 25 mg t.i.d. for 28 days. At an outpatient clinic, each patient was followed up regularly for efficacy, compliance and possible adverse events. Both drugs produced a statistically significant improvement for primary efficacy variables: change of pain score during motion and restriction of body movement. Values for overall tolerability and incidence of gastrointestinal side effects were significantly lower for acemetacin than for indomethacin. We therefore suggest that acemetacin, by demonstrating significant tolerability and safety advantages, is as effective as indomethacin for the treatment of OA.