The activities of hepatic lipase and of lipoprotein lipase, the elimination rate of exogenous triglyceride and the cholesterol esterification rate were determined and related to plasma lipoprotein concentrations in 16 patients before and after treatment for hyperthyroidism. The activity of hepatic lipase was significantly higher (65%) before than after treatment, while the activity of lipoprotein lipase and the elimination rate of exogenous triglyceride remained unchanged. The endogenous cholesterol esterifying ability decreased after treatment, whereas no change occurred in the fractional cholesterol esterification rate measured with normal plasma as substrate. The concentrations of LDL-cholesterol and HDL-cholesterol increased significantly after treatment. The decrease in hepatic lipase activities was correlated to the decrease in S-T3 concentrations (r = 0.77, P less than 0.001) and to the increase in HDL-cholesterol concentrations (r = 0.51, P less than 0.05). The activities of lipoprotein lipase were positively correlated to the concentrations of HDL-cholesterol both before (r = 0.54, P less than 0.05) and after (r = 0.59, P less than 0.05) treatment. These results support the view that hepatic lipase and lipoprotein lipase are both important determinants of plasma HDL concentrations and suggest that an increased hepatic lipase activity contributes to the lower HDL levels in hyperthyroid patients.