Hyperglycaemia has been reported to cause increased production of oxygen free radicals. Oxidative stress may contribute to the pathogenesis of diabetic complications. Coenzyme Q(10) (CoQ(10)) is known for its key role in mitochondrial bioenergetics and is considered as a potent antioxidant and free radical scavenger. This study was conducted to evaluate plasma and blood cell concentrations of CoQ(10) in accordance to its redox capacity in children with diabetes mellitus type 1. CoQ(10) plasma and blood cell concentrations and redox status were measured using high-performance liquid chromatography with electrochemical detection in 43 children with diabetes mellitus type 1 and compared with 39 healthy children. In addition, the diabetic patients were subdivided according to their haemoglobin A1c (HbA1c) values into two groups, that is, those with good control (<8%) and those with poor control (>8%), and the CoQ(10) status was compared between the two groups. Children with type 1 diabetes showed increased plasma levels of CoQ(10) in comparison to healthy children. While CoQ(10) erythrocyte and platelet concentrations did not differ, in the diabetes group, the platelet redox status differed with a significantly increased part of reduced CoQ(10). This difference in concentration and redox status in comparison to healthy controls may be attributed to the subgroup of patients with poor control, as the subdivision of diabetic patients according to their HbA1c values shows. In diabetic children, especially in those with poor control, an increase in plasma concentration and intracellular redox capacity of the antioxidant CoQ(10) may contribute to the body's self-protection during a state of enhanced oxidative stress.