Total fat balance was studied in 13 healthy controls and 14 patients with primry hyperlipidemia by estimating dietary and fecal fat. Although no difference in fat intake was noted between hyperlipidemics and controls, fecal fat excretion in hyperlipidemics increased significantly as compared with that in controls. The increase in fat excretion was greater in patients with type IIa hyperlipidemia (hypercholesterolemia) than in those with types IIb and IV hyperlipidemia (endogenous hyperlipidemia).