Serial measurement of liver enzymes is useful to detect liver toxicity due to methotrexate in patients with rheumatoid arthritis or other rheumatic diseases. We have reviewed retrospectively 141 adult patients treated with methotrexate from 1988 to 1991. The more common diagnoses included rheumatoid arthritis (120 cases) and psoriatic arthritis (12 cases). In periodic studies carried our every 2-3 months, a transient increase in transaminase values associated with methotrexate in 13 patients (9.2%) was observed. Two patients developed a viral infection during therapy, one due to cytomegalovirus and the other due to the Epstein-Barr virus. Both patients had a favorable outcome once methotrexate was withdrawn.