In a multicentre trial, tiaprofenic acid was administered in a dosage of 600 mg/day for 12 months to 109 patients with rheumatoid arthritis. The results in 79 patients, who did not receive any other non-steroidal anti-inflammatory drugs concomitantly, were assessed. Of the 79 patients, 11 withdrew from the study within one year. The reasons for withdrawal were personal reasons in 4 cases, side effects or abnormal laboratory values in 2, no change or aggravation of symptoms in 2, improvement of symptoms in 2, and identification of systemic lupus erythematosus in one. Overall improvement was definite in 27 patients (34.2%) and slight in 21 patients (26.6%). Overall usefulness was definite in 30 patients (38%) and slight in 27 patients (34.2%). The duration of morning stiffness, joint index and Lansbury activity index showed significant improvement after 6 months' administration. Side effects were observed in 2 cases (2.5%) and abnormal laboratory results were seen in 3 (3.8%). These disappeared on either continuation or discontinuation of treatment.