Three South African blacks with hepatocellular carcinoma and arterial hypertension are described. Plasma angiotensinogen (renin substrate) concentrations were increased eightfold to 10-fold in the two patients in whom these concentrations were measured. One of these two patients also showed a 34-fold rise in plasma inactive, active, and total renin concentrations, and an elevated plasma renin activity (2.73 ng.L-1.s-1 angiotensin l/mL/h). Inactive renin (prorenin) constituted 90% of the total plasma renin concentration. In the third patient only plasma renin activity was measured, and this was considerably raised (6.05 ng.L-1.s-1; angiotensin l/mL/h). Thus, the arterial hypertension that rarely complicates hepatocellular carcinoma may be caused either by a combination of eutopic synthesis of excessive quantities of angiotensinogen and ectopic production and secretion of active renin by malignant hepatocytes, or by eutopic production of angiotensinogen alone.