Liver function was studied primarily by determination of serum gamma glutamyl transferase and alkaline phosphatase. In subsamples of patients the investigation was extended by determination of serum amino-transferases, isoenzyme analysis of alkaline phosphatase, 99mtechnetium scintigraphy, and liver biopsy. In 183 in-patients with rheumatoid arthritis, the serum gamma glutamyl transferase level was elevated in 47% and serum alkaline phosphatase (of liver origin) in 24%. A concomitant increase in serum aminotransferases was found in 15% of patients with elevated gamma glutamyl transferase level. A closely similar pattern was found in 45 patients with non-rheumatoid arthritis (ankylosing spondylitis, psoriatic arthritis, reactive arthritis, and undefined arthritis), and in 5 patients with polymyalgia rheumatica. In 23 patients with non-rheumatic inflammation (pneumonia), liver dysfunction was common, though the pattern of serum enzyme changes was different. In rheumatoid arthritis, liver scanning showed irregular or low uptake, but biopsy only indicated reactive hepatitis. Hepatotoxicity could not be traced to any single drug or combination of drugs given. On the contrary, chloroquine appeared to reduce serum gamma glutamyl transferase, and corticosteroids had a similar effect on serum alkaline phosphatase. In patients not treated with corticosteroids, both serum gamma glutamyl transferase and alkaline phosphatase were weakly to moderately correlated with laboratory indices of disease activity (ESR and serum orosomucoid). The frequently occurring isolated increase of serum gamma glutamyl transferase and/or serum alkaline phosphatase in arthritis may be an unspecific reaction to inflammation.