Abstract In this survey we have studied the results of several liver function tests, particularly the thymol, cephalin, prothrombin, Bromsulphalein, albumin/globulin ratio and alkaline phosphatase tests. Most of the diseases studied were surgical, but some important non-surgical diseases such as viral hepatitis and cirrhosis have been included. In patients with cirrhosis the three important features were: (1) Bromsulphalein retention in 95 per cent, with the majority being marked; (2) a reversed albumin/ globulin ratio in 53 per cent; and (3) an elevated thymol turbidity in 81 per cent. Cirrhotic patients with varices were found to have a higher incidence of hyperglobulinemia than those in the cirrhotic group as a whole. Patients with portal vein thrombosis were included because they often present indirectly with bleeding esophageal varices, and the differential diagnosis arises with the more common etiologic factor, cirrhosis. The liver function tests in this study clearly separated these patients from those with cirrhosis, since test results were normal, with the exception that two-thirds of the patients with venous thromboses had an abnormal prothrombin time. Like the other major category of hepatic parenchymal disease (cirrhosis), all patients with viral hepatitis had abnormal tests. The most significant feature was the fact that the thymol test was elevated in 98 per cent of the patients and markedly so in two-thirds; in 88 per cent of the patients in this group the cephalin flocculation test was positive. Only 7 per cent had an abnormal albumin/globulin ratio. Minimal elevations of the alkaline phosphatase were common. The primary neoplasms of the liver, most of which were carcinomas, were associated in four instances with coexisting cirrhosis. The only constant observation was that all patients tested had significant Bromsulphalein retention, most to a marked degree. Hypoalbuminemia was common. In about half, the prothrombin test and either the thymol or cephalin tests were positive. In metastatic hepatic neoplastic disease the most prominent finding was Bromsulphalein retention in seven of ten patients tested. In five of nine patients the alkaline phosphatase test was abnormal, but usually to a minimal degree. The eighty-four patients with extrahepatic obstruction of the biliary tract from all causes can be considered together because they had similar tests results, the type of obstruction causing variation only in degree. The single most noteworthy finding was an 89 per cent incidence of alkaline phosphatase elevation, associated with only a 5 per cent occurrence of cephalin flocculation abnormality. Popper has mentioned this finding and it appears to be a significant one. Table II considers all patients with obstructive disease as a single group and permits comparison with the two major patient groups having primary parenchymal disease. Several additional facts are apparent from this study: (1) In no group of patients was the cephalin flocculation abnormal as often as the thymol turbidity. This fact is indicative of the comparatively greater sensitivity of the latter over the former. In our patients with primary parenchymal disease both these tests were positive in a high incidence, but in patients with obstructive disease the cephalin flocculation was uncommonly positive and the thymol only occasionally so. (2) The Bromsulphalein test was noted to be of little diagnostic value in the presence of jaundice, almost all such patients having predictable retention. (3) A significant incidence of prothrombin time abnormality was noted in every type of disease process considered and was, therefore, of very little diagnostic value. From these data we conclude that liver function tests have been of definite diagnostic value to us, particularly in differentiating intrahepatic disease from extrahepatic obstruction. The thymol was a reliable test of hepatic parenchymal disease. The cephalin flocculation was found to be of great value in excluding primary parenchymal disease in patients with obstruction of the extrahepatic biliary system. Serum alkaline phosphatase values were evaluated in at least three-quarters of patients with extrahepatic obstruction. The Bromsulphalein test was the most frequently positive test in cirrhosis and was commonly so in hepatic neoplastic disease.