Plasma low density lipoprotein (LDL) kinetics and their relation to plasma very low density lipoprotein (VLDL) and LDL composition were determined in patients with familial combined hyperlipidemia (FCHL) of varying lipoprotein phenotypes. In both Type II and IV subjects, LDL apolipoprotein B (apo B) synthesis was greater than normal. In Type IV, the VLDL triglyceride/apo B ratio was normal and almost all of the LDL apo B was derived from VLDL. LDL cholesterol/apo B ratio was diminished and LDL apo B fractional catabolic rate (FCR) was sufficiently increased to prevent a rise in plasma LDL concentration. In Type II, VLDL triglyceride/apo B was reduced and 14% to 50% of the LDL was formed by direct synthesis. LDL cholesterol/apo B was normal and LDL apo B FCR was lower than in Type IV subjects. In four patients whose plasma lipid levels became normal with carbohydrate restriction and intake of a fibric acid derivative, plasma VLDL and LDL composition and LDL kinetic measurements remained unchanged. By contrast, VLDL triglyceride-apo B decreased and direct LDL synthesis increased in four patients whose phenotype changed to Type IIa. LDL cholesterol/apo B also increased and LDL FCR declined. Among patients with FCHL, significant correlations between VLDL triglyceride/apo B, LDL apo B derived by direct synthesis, LDL cholesterol/apo B, and LDL apo B FCR were found. Thus, increased apo B synthesis is a characteristic feature of FCHL. The phenotypic expression is determined by the availability of triglyceride for hepatic coupling to apo B which could influence the source, composition, and removal rate of circulating LDL. Despite normalization of their plasma lipid levels, some patients continued to show the compositional and kinetic features of FCHL.