ABSTRACT : A dose-escalating phase II trial studied masitinib, an oral tyrosine kinase inhibitor, in 43 patients with rheumatoid arthritis. Masitinib induced American College of Rheumatology (ACR)20, ACR50 and ACR70 responses in 54%, 26% and 8% of patients, respectively. A placebo group was not included. Thirty-seven per cent of the patients withdrew before the 12-week end-point was reached, primarily because of adverse events. These findings are the first on the efficacy of tyrosine kinase inhibition in a sizeable population. Future work should focus on delineating the tyrosine kinase that is most important in maintaining rheumatoid activity and address potential long-term toxicities such as gonadal insufficiency, teratogenicity and cardiotoxicity.