A patient is described with definite rheumatoid arthritis (RA) who developed life threatening acute pneumonitis after receiving a total dose of only 12.5 mg methotrexate (MTX). This complication has been previously described, but this is probably the lowest reported dose before development of pneumonitis in a patient with RA. The possible significance of this case is discussed in the light of recent reports suggesting an increased susceptibility of patients with RA to the pulmonary toxicity of MTX.