High-dose leucovorin (folinic acid) supplementation was tested in a prospective, unblinded manner for 4 weeks in 7 rheumatoid arthritis patients who were being treated successfully with low-dose methotrexate (MTX). Nausea caused by MTX disappeared; however, the underlying rheumatic disease worsened in all patients. Subjective clinical assessment, Ritchie articular index, grip strength, erythrocyte sedimentation rate, and levels of C-reactive protein showed statistically significant deterioration. All these parameters improved after the leucovorin was stopped. This is the first direct clinical evidence implying folate antagonism in the action of low-dose MTX therapy in rheumatoid arthritis patients.