Familial combined hyperlipidemia (FCHL) is a heterogeneous disorder characterized by multiple lipoprotein phenotypes, a high risk for coronary heart disease, and predominance among the LDL fraction of smaller and denser particles. We report on an FCHL kindred (the M-kindred) in which decreased VLDL- and LDL-apoB elimination rates rather than enhanced production rates were the main kinetic abnormalities. Lipoprotein levels and metabolic parameters of all apoB-containing lipoproteins (including light and dense LDLs) were determined during placebo and pravastatin treatment periods. ApoB metabolism was studied by endogenous labeling with stable isotopes and a multicompartmental model. Five members of the M-kindred participated. The study was doubly blinded, randomized, and placebo controlled. Treatment periods of 6 weeks were separated by 2-week washout periods. All subjects had high apoB levels, 2 had a mixed lipemia, 1 had hypercholesterolemia, and 2 had hypertriglyceridemia. Familial dysbetalipoproteinemia, hypercholesterolemia, and defective apoB-100 were excluded by genetic, testing. Kinetic parameters were remarkably similar in the five study subjects during the placebo period, despite their diverse plasma lipid profiles. Compared with nine normolipidemic control subjects, low VLDL-apoB fractional catabolic rates (FCRs) (3.6 +/- .1 versus 9.3 +/- 2.9 pools per day) and low LDL-apoB FCRs (0.19 +/- 0.05 versus 0.41 +/- 0.13 pool per day) were observed in every case. The majority of the LDL particles were identified in the denser fraction (d = 1.036 to 1.063 g/mL). A clear precursor-product relationship was observed from VLDL to IDL to light LDL to dense LDL, ie, there was no "metabolic channeling." Light LDL had significantly higher FCR than dense LDL (0.82 +/- 0.21 versus 0.22 +/- 0.08 pool per day). VLDL-apoB production rates were normal (19.7 +/- 6.0 versus 21.6 +/- 6.1 mg/kg per day for control subjects). In contrast, in two subjects drawn from two other FCHL kindreds (the C- and K-kindreds), VLDL-apoB production rates were increased (35.6 and 32.1 mg/kg per day, respectively). In these two, more "typical" FCHL subjects, FCRs of LDL-apoB were near normal (0.351 and 0.311 pool per day, respectively). Pravastatin (20 mg/d) resulted in significantly lower plasma cholesterol (265 +/- 30 to 218 +/- 16 mg/dL, P < .01), LDL cholesterol (186 +/- 31 to 145 +/- 15 mg/dL, P < .03), and apoB levels (168 +/- 14 to 125 +/- 16 mg/dL, P < .01) in the five FCHL subjects of the M-kindred. No changes were observed in plasma HDL cholesterol, apoA-I, or lipoprotein(a). Pravastatin significantly increased the LDL-apoB FCR (from 0.19 +/- 0.05 to 0.34 +/- 0.04 pool per day). The FCRs of both LDL subclasses increased with treatment. No pravastatin-induced changes were seen in apoB production rates.