Decreased serum ubiquinone-10 concentration is a common condition in patients with phenylketonuria (PKU) under dietary treatment. Our aim was to investigate the implication of the metabolic abnormalities of PKU (low concentrations of tyrosine and high concentrations of phenylalanine) and the effect of treatment with phenylalanine-restricted diets in decreased ubiquinone-10 concentrations in PKU patients. We studied 30 PKU patients (age range 5 months to 35 years; median age 7 years) under dietary treatment. Correlation between plasma tyrosine or phenylalanine and serum ubiquinone-10 concentrations was investigated. Daily cholesterol intake was calculated from the data obtained through a dietary questionnaire. The index of dietary control (IDC) was calculated as the average of the medians of plasma phenylalanine concentrations obtained every 6 months in the metabolic control of patients. Negative correlations were observed between serum ubiquinone and the IDC (r=-0.46; p<0.01) in PKU patients. No correlation was observed between tyrosine or daily cholesterol intake and serum ubiquinone concentrations. After adjustment for daily cholesterol intake by multiple linear regression analysis, for each 113 units of increase in IDC values serum ubiquinone decreased 0.1 μmol/L. According to our results, the main factor associated with the decreased serum ubiquinone concentrations was high plasma phenylalanine values. Although daily cholesterol intake seems to be associated with ubiquinone concentrations, it may not be relevant if we take into account the low intake of cholesterol in treated PKU patients.