Pirazolac is a pyrazole-acetic acid derived from indomethacin. It is a nonsteroidal anti-inflammatory agent. Pirazolac concentration-relatedly inhibited the accumulation of prostanoids in incubates of human gastric mucosa, but this inhibition was less than that by indomethacin and other commonly used non-steroidal anti-inflammatory drugs. Pirazolac was at least as effective as sulindac in the treatment of patients with rheumatoid arthritis. The abnormal activity of neutrophils from patients with rheumatoid arthritis was partially corrected with pirazolac. Patients with ankylosing spondylitis benefited similarly from treatment with pirazolac and indomethacin. Pirazolac was developed in an effort to overcome the adverse gastrointestinal effects of indomethacin. However, more patients with ankylosing spondylitis withdrew from treatment because of intolerable adverse events with pirazolac than with indomethacin in a clinical trial. In patients with rheumatoid arthritis, pirazolac had similar tolerability to sulindac.