1. The plasma lipoprotein profiles of eight members of a Dutch pedigree spanning three generations where two rare apolipoprotein E mutants, APOE*3(Cys-112-->Arg; Arg-251-->Gly) and APOE*2(Val-236-->Glu), segregate were analysed to determine whether the APOE mutants were associated with dyslipidaemia. 2. The proband, a 51-year-old Caucasian male, was a carrier of APOE*3(Cys-112-->Arg; Arg-251-->Gly) and his spouse was a carrier of APOE*2(Val-236-->Glu). Four other family members were carriers of one or both of the mutant APOE genes. 3. The plasma cholesterol and triacylglycerol concentrations were markedly elevated in the proband and were classified as type IV hyperlipoproteinaemia. The plasma triacylglycerol concentration was moderately increased in a sister, who was a carrier of APOE*3(Cys-112-->Arg; Arg-251-->Gly), and in the son, who was a compound heterozygote for both mutant APOE alleles. Normal plasma lipid levels were observed in all other family members. In the plasma samples of the proband and his family members beta-very-low-density lipoprotein was not detectable and the molar ratio of very-low-density lipoprotein-cholesterol to very-low-density lipoprotein-triacylglycerol was less than 0.9. The concentration of intermediate-density lipoprotein was within normal limits. 4. None of the family members carrying APOE*3-(Cys-112-->Arg; Arg-251-->Gly) and/or APOE*2(Val-236-->Glu) exhibited lipoprotein abnormalities characteristic of familial dysbetalipoproteinaemia, although three family members carrying APOE*3-(Cys-112-->Arg; Arg-251-->Gly) showed hypertriglyceridaemia.