Mixed hyperlipidaemia is a common finding in glycogen storage disease type Ia (GSD Ia). Although cross-sectional studies have demonstrated increases in intermediate-density lipoproteins (IDLs) and reductions in lipoprotein lipase activity, no studies have investigated the dynamics of apolipoprotein B-100 (apo B) metabolism in GSD Ia. This study investigated apoB turnover in GSD Ia using an exogenous labelling method in one sib from a kinship with established GSD Ia. The study demonstrated normal hepatic secretion of very low-density lipoprotein (VLDL), but hypocatabolism of VLDL, probably due to lack of lipoprotein lipase activity. The production rate of IDL was slightly increased, but the turnover rate of low-density lipoprotein was normal. The findings suggest that, as well as a corn starch diet and dietary fat restriction, treatment of severe mixed hyperlipidaemia in GSD Ia and its attendant risk of pancreatitis should possibly involve fibrates that activate lipoprotein lipase and may enhance the clearance of IDL, rather than omega-3 fatty acids, which principally suppress hepatic secretion of VLDL.