Oxidative mechanisms involving lipid peroxidation in the subendothelium of the arterial vessel wall play a key role in atherogenesis. Despite severe hyperlipidaemia, patients with glycogen storage disease type 1a (GSD1a) do not develop premature atherosclerosis. Therefore, we analysed parameters of antioxidative defence and oxidative stress in plasma and serum of patients with GSD1a ( n=17) and compared them with those of patients with type 1 diabetes mellitus ( n=17), familial hypercholesterolaemia ( n=18) and healthy controls ( n=20). We measured the total radical trapping ability parameter (TRAP), single plasma antioxidants (sulfhydryl-groups, uric acid, vitamin C, alpha-tocopherol, coenzyme-Q10), markers of lipid peroxidation, lipoprotein (a) and homocysteine. Patients with GSD1a showed an elevated TRAP ( P<0.01) compared to the three other groups. This can mainly be attributed to elevated uric acid levels ( P<0.05 versus control). Lipoprotein (a) was significantly lower in the GSD1a group compared to the three other groups ( P<0.05). CONCLUSIONS Patients with glycogen storage disease type 1a show an increased antioxidative defence in plasma which may protect them against lipid peroxidation and thus against premature atherosclerosis. Our finding of low lipoprotein(a) levels in this small group of patients warrants further investigation in a greater number of patients before assessing its role in atherogenesis in glycogen storage disease type 1a.