Levels of serum glutamic oxaloacetic transaminase (SGOT) and serum glutamic pyruvic transaminase (SGPT) in patients with rheumatoid arthritis from 5 centers involved in the Arthritis, Rheumatism, and Aging Medical Information System were correlated with the use of specific antirheumatic medications. Elevated levels of SGOT and SGPT were most frequent in patients taking salicylates and methotrexate (MTX) and least frequent in patients taking hydroxychloroquine. The combination of MTX and salicylates greatly increased the frequency of abnormal liver enzyme values. In contrast, the addition of hydroxychloroquine to a regimen of either MTX or aspirin essentially eliminated the SGOT and SGPT abnormalities. Results from all 5 centers were consistent and remained so after adjustment for age, sex, and disease duration. Knowledge of these important drug interactions may permit continuation of MTX therapy in patients in whom the drug might otherwise be discontinued.