We have studied the large nonconsanguineous pedigree of a proband with Type I hyperlipoproteinemia (HL) and lipoprotein lipase (LPL) deficiency. Within the nuclear family, the mother and two of the proband's five siblings had fasting hypertriglyceridemia or low-normal tissue adipose LPL activities or both. Retention of lipoprotein retinyl esters after vitamin A feeding was present only in the propositus. The maternal side of the extended pedigree contained individuals with Types IIA, IV, and V hyperlipoproteinemia, findings most consistent with autosomal dominant multiple lipoprotein-type hyperlipidemia (familial combined hyperlipidemia). This family and previously reported pedigrees of Type I HL probands have demonstrated phenotypic heterogeneity. Without specific genetic markers, homozygous LPL deficiency and complex multiple-gene mechanisms cannot be distinguished unambiguously. Parental hyperlipidemia in nuclear pedigrees of Type I HL probands should not be equated with heterozygous LPL deficiency in the absence of extended pedigree data or more informative markers. The possibility that the complex inheritance of two different genetic defects in lipoprotein transport can produce the Type I HL phenotype must be considered.