A six-month double-blind, randomized, parallel-design trial comparing pirprofen, a new nonsteroidal antiinflammatory drug, with aspirin in patients with rheumatoid arthritis is described. The dose of pirprofen, a phenylpropionic acid derivative, was 600 mg per day; the dose of aspirin was 3600 mg per day. Pirprofen was found to be as effective as aspirin in improving the modified articular index, swelling joint index, grip strength, walking time, and duration of morning stiffness. Fewer patients receiving pirprofen reported drug-related side effects than those receiving aspirin, however, the difference was not statistically significant (P less than 0.1). There was no significant difference between groups in the incidence of positive stool guaiac tests (three of 17, pirprofen; five of 18, aspirin). Pirprofen was found to be as safe and effective as aspirin in the management of patients with rheumatoid arthritis.