We investigated whether 5 to 20mg per week oral methotrexate could slow down disease progression in 44 patients with inclusion body myositis in a randomized double-blind placebo-controlled study over 48 weeks. Mean change of quantitative muscle strength testing sum scores was the primary study outcome measure. Quantitative muscle strength testing sum scores declined in both treatment groups, −0.2% for methotrexate and −3.4% for placebo (95% confidence interval = −2.5% to +9.1% for difference). There were also no differences in manual muscle testing sum scores, activity scale scores and patients' own assessments after 48 weeks of treatment. Serum creatine kinase activity decreased significantly in the methotrexate group. We conclude that oral methotrexate did not slow down progression of muscle weakness but decreased serum creatine kinase activity.