Bile acid kinetics were determined in 15 normolipidemic and 61 hyperlipidemic subjects with the aid of [¹⁴C]cholic acid and [³H]chenodeoxycholic acid. The diet was standardized and of natural type. The total bile acid formation was within normal limits in patients with hyperlipoproteinemia types IIa and IIb. On the average the production of cholic acid (C) represented less than 50% of the total bile acid synthesis in both groups. The corresponding value recorded for the controls was 64±2% (mean±SEM). The synthesis of C in hyperlipoproteinemia type IIa was significantly below normal. Of the 27 patients with the type IV pattern, 18 had a synthesis of C and C + chenodeoxycholic acid (CD) that exceeded the upper range recorded for the controls. In these subjects the C formation represented 73±3% of the total bile acid synthesis. Similar findings were also encountered in the five patients with the type V lipoprotein pattern studied. The bile acid pool size of the 11 patients with hyperlipoproteinemia type IV, who had been cholecystectomized or suffered from cholelithiasis, was 900 mg smaller on the average than that of the other subjects with the same type of hyperlipoproteinemia. However, the pool size in the former subjects still tended to be higher than that of the control subjects without evidence of gallbladder “disease”. In all groups of subjects the formation of bile acids tended to be higher in the male than in the female subjects. Bile acid synthesis showed no linear correlation to actual body weight, relative body weight, or body surface area. A moderate weight reduction in five patients (one with type IIb and four with type IV pattern) was followed by a 50% reduction of the C and CD synthesis.