Serum gamma-glutamyl transferase (GGT, EC. 2.3.2.2. was measured in 173 patients with diseases of the hepatobiliary system (including metastatic cancer) and in 90 patients who were subsequently shown to have primary diseases of other etiology. All patients had been selected because they had abnormal alkaline phosphatase, aspartate aminotransferase or bilirubin on SMA 12/60 screening. Serum GGT was elevated in 97% of patients with primary hepatobiliary disease. The magnitude of the increase in GGT was variable in all groups and was unhelpful in differential diagnosis, even between medical and surgical cases. Moreover, GGT was abnormal in 69 patients who did not have primary hepatobiliary disease (77%), an incidence higher than that for other enzyme tests performed. We conclude that because GGT was more susceptible than other tests to spurious elevation in the absence of hepatobiliary disease and was unhelpful in differential diagnosis, it has little value apart from monitoring alcohol abuse and enzyme induction.