The specificity of the serum gamma-glutamyl transpeptidase (GGT) was evaluated by its determination in 1,040 unselected adult inpatients. GGT was elevated in 139 (13.4%) patients, but was only rarely elevated in the absence of elevation of other enzymes in the routine chemistry profile. Of the elevations, 32.4% occurred in patients with primary hepatobiliary disease. An elevated serum GGT is a strong indicator of hepatobiliary dysfunction. However, proper interpretation of a serum GGT elevation requires correlation with clinical data and other tests.