Eighteen patients with chronic inflammatory joint disease were treated for two successive five day sequences, in a double blind crossover study, with 300 mg per day ketoprofen given alone or combined with 1,500 mg aspirin. Bioavailability and pharmacokinetics of total serum ketoprofen studied in eight patients did not appear to be significantly modified by addition of aspirin. Measurements of Ritchie's articular index and Lee's functional index demonstrated that ketoprofen was clinically as effective when given alone as in association with aspirin. Measurement of the sedimentation rate by the sigma ESR technique showed that aspirin produced a marked reduction of the antiinflammatory biological activity of ketoprofen which did not however reach statistical significance. This study of interactions between ketoprofen and aspirin shows that it is useless and perhaps harmful to combine the two medications.