Studies were carried out on cholesterol metabolism in 11 nonobese patients and 16 obese patients with hypothyroidism and 13 with hyperthyroidism. The patients underwent several investigations under metabolic ward conditions. Hypothyroid patients usually had an increase in low density lipoprotein (LDL)-cholesterol. Several mechanisms may have combined to cause a high LDL. For instance, the obese hypothyroid patients had an increase in cholesterol synthesis. Absorption of cholesterol also was increased frequently. However, other mechanisms not explored in this study probably contributed to most of the fall in LDL-cholesterol. Treatment of hypothyroid patients produced the expected fall in LDL. One possible mechanism could be that thyroid hormones enhance the conversion of cholesterol into bile acids; this mechanism has been suggested by other workers from animal studies. However, no evidence was obtained in either hypothyroid or hyperthyroid patients that thyroid hormones alter synthesis of bile acids. On the other hand, the hormones appeared to increase the synthesis of cholesterol. Patients with hypothyroidism frequently had supersaturated bile. The cause was mostly an enhanced secretion of biliary cholesterol associated with a tendency to obesity and increased synthesis of cholesterol. In contrast, the usually thin hyperthyroid patients did not have supersaturated bile. The studies show that thyroid hormones a) influence LDL-cholesterol by an action on the catabolism of LDL-independent of alterations in synthesis, catabolism, absorption, or excretion: b) stimulate synthesis of cholesterol; and c) affect biliary lipid metabolism in large part by influencing energy balance and cholesterol synthesis.