The present study was undertaken to determine the relationship between plasma lecithin: cholesterol acyl transfer (LCAT) rate and cholesterol and bile acid turnover in hyperlipidaemia. Nineteen healthy controls, 19 patients with hyperlipoproteinaemia (HLP) type IIa and 12 patients with HLP type IV were studied under standardized dietary conditions. Bile acid kinetics was determined with the aid of [¹⁴C]labelled cholic acid and chenodeoxycholic acid. In the hyperlipidaemic patients, cholesterol balance was calculated as the sum of bile acid synthesis plus daily faecal excretion of neutral C₂₇ steroids minus dietary intake of cholesterol. The plasma LCAT rate was determined simultaneously. The mean values of bile acid formation, cholesterol balance, and LCAT rate in HLP type IV patients exceeded those in HLP type IIa patients or in the controls. An increased plasma LCAT rate was found among HLP type IV patients with and without evidence of cardiovascular disease. Plasma LCAT rate correlated positively with bile acid formation (R_s = +0.78, p<0.01) and cholesterol balance (R_s = +0.88, p<0.002) in HLP type IV. No such relationships were obtained in the controls or in HLP type IIa. It is suggested that an increased production and/or flux of VLDL in HLP type IV is linked to an enhanced plasma LCAT rate and to an increased formation and metabolism of cholesterol in the liver.