Objectives: To investigate the ubiquinone-10 content in lymphocytes from phenylketonuric patients. Design and methods: We compared 23 patients with 25 age-matched controls. Ubiquinone-10 was analyzed by HPLC with electrochemical detection. Results: Ubiquinone-10 concentrations were significantly lower in patients (77-270 nmol/g of protein) compared with controls (190-550) (p < 0.001). Significantly negative correlation was observed between ubiquinone-10 and phenylalanine (r = -0.441; p < 0.05). Conclusions: Ubiquinone-10 concentrations are decreased in lymphocytes from phenylketonuric patients. This deficiency is associated with high plasma phenylalanine concentrations.