Treatment with an investigational antibody targeting granulocyte–macrophage colony-stimulating factor (GM-CSF) improved the signs and symptoms of rheumatoid arthritis (RA) in a dose-finding phase II randomized controlled trial. Patients with RA and an inadequate response to methotrexate (n = 92) or anti-TNF therapy (n = 16) received subcutaneous namilumab (20 mg, 80 mg or 150 mg) or placebo at baseline and at weeks 2, 6 and 10. At week 12, disease activity was significantly reduced from baseline in patients treated with all doses of namilumab versus placebo (P = 0.005), with a dose–response effect apparent from week 2.