T his study was designed to examine the integrated metabolism of apolipoprotein B (apo B) in very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low density lipoproteins (LDL) in normal subjects, obese patients, and a group of patients with coronary heart disease (CHD). Turnover rates of 131I-VLDL-B, 131I-IDL-B, 125I-LDL-B, and [3H]VLDL-triglycerides (TG) were determined by the multicompartmental analysis that used the model described in the preceding article (Beltz, W.F., et al. 1985. J. Clin. Invest. 76: 575-585). Compared with five normal subjects, four obese subjects had increased synthesis rates of both VLDL-B and VLDL-TG. Production of LDL-B was inconsistently raised in these same patients. Five patients with CHD had enhanced production of both VLDL-B and LDL-B, but secretion rates of VLDL-TG were not increased. Thus, in patients with obesity and in those with CHD, synthesis rates of VLDL particles may be abnormally high. In the obese patients, the VLDL appeared to be of normal composition, but in patients with CHD, the VLDL were relatively poor in TG. The study also showed that a significant fraction of VLDL-B is removed directly from the circulation and never reaches LDL regardless of the type of patients. The fraction that does reach LDL is one factor that determines LDL concentrations.