Tolfenamic acid (TA) is an interesting drug for the treatment of rheumatic diseases because of its capacity to inhibit the synthesis of leukotrienes. It may have fewer upper gastrointestinal side effects than other NSAIDs which inhibit only the synthesis of prostaglandins. Several controlled and non-controlled studies on the clinical efficacy and side effects of TA have been carried out since the early seventies. These studies include about 900 patients suffering from different rheumatic diseases. The clinical efficacy of TA has proved to be at least as good as that of control drugs in all double-blind trials. We have compared the analgesic effect of ten NSAIDs in patients with rheumatoid arthritis, using a single-blind method by asking the patients which one of two drugs was the better. The study gave a rank order to the drugs favoured by the patients and the results showed that TA was among the four best drugs together with naproxen, indomethacin, and diclofenac. The side effect profile of TA is different from that of other NSAIDs. The number of upper gastrointestinal side effects during TA treatment was less than half the number during treatment with control NSAIDs in eight double-blind studies. On the other hand, dysuria was found only during TA treatment. In 1989 the official side effect registers of Denmark and Finland included a total of 462 side effect reports. The frequency of side effects per treatment day was about the same as for other NSAIDs according to these reports.(ABSTRACT TRUNCATED AT 250 WORDS)