Plasma lipoproteins, triglyceride turnover, and lipolytic enzymes were measured in 11 women with Cushing's syndrome. The studies were repeated 3 and 12 months after surgical treatment. Eleven healthy women of similar age and relative body weight served as controls. Before treatment the mean values of total cholesterol and triglyceride, of very low-density lipoprotein (VLDL) triglyceride and cholesterol, low density lipoprotein triglyceride and cholesterol, and high density lipoprotein cholesterol were all significantly increased in the patients with Cushing's syndrome. The triglyceride levels were only moderately elevated, the highest values being found in patients with adrenocortical adenoma. The production rate of VLDL triglyceride was higher in patients (13.2 mg/h . kg) than in controls (9.5 mg/h . kg, P less than 0.05), whereas the fractional catabolic rate of VLDL triglyceride was not significantly different. Consistent with the latter finding, the lipoprotein lipase activities of adipose tissue, skeletal muscle, and postheparin plasma were similar in patients and controls. The postheparin plasma hepatic lipase activity of the patients was at the lower end of the normal range. All lipid and lipoprotein abnormalities were completely abolished after successful surgery. It is concluded that endogenous hypercortisolism stimulates the hepatic production of VLDL particles. The effect is probably based on multifactorial mechanisms. In the presence of unchanged removal this leads to elevated levels of VLDL, low density lipoprotein, and high density lipoprotein.