ABSTRACT Klinefelter syndrome (KS) is a genetic condition affecting 1 in 600 males who have an additional X chromosome (47,XXY) associated with multisystem manifestations and increased mortality secondary to disorders of insulin resistance. One of the hallmark features of KS is primary testicular failure resulting in hypogonadism, and to date androgen treatment has been the only therapeutic intervention studied in these individuals. However, insulin resistance and abnormal metabolism have been observed in youth with KS prior to the onset of testicular hypogonadism. Furthermore, testosterone replacement does not ameliorate these cardiometabolic deficits that the majority of these individuals experience. The underlying molecular mechanisms for the high prevalence of insulin resistance and exercise intolerance observed in KS are unknown. This application proposes that the additional X chromosome leads to metabolic aberrations that can be targeted for therapeutic intervention. Peroxisome proliferator-activated receptor alpha (PPAR-α) is transcription factor that regulates the expression of genes controlling fatty acid metabolism, inflammation, and oxidative stress. Low PPAR-α activity is associated with cardiometabolic profiles similar to that observed in KS, including adiposity, dyslipidemia, and exercise intolerance. Our preliminary data have shown a metabolome and transcriptome consistent with insufficient activity of the PPAR-α complex in males with KS. The central hypothesis of this study is that lower PPAR-α activity contributes to the cardiometabolic phenotype in KS and that increasing PPAR-α activity via PPAR-α agonist treatment will result in upregulation of gene transcription that will improve cardiometabolic physiology. In this proof-of-concept trial, we will first compare expression of PPAR-α-regulated genes from whole blood and skeletal muscle in adolescent and young adult males with and without KS, as well as systemic fat oxidation during submaximal prolonged exercise and tissue-specific mitochondrial ß-oxidation. Resting energy expenditure, metabolite concentrations, and patient-centered outcomes will also be obtained and compared between groups. The KS cohort will then receive intervention with a PPAR- α agonist (fenofibrate) for one month, and all outcomes will be reassessed. This study will lay the foundation for future investigation of the first ever non-androgen treatment to improve insulin resistance and exercise intolerance in males with KS. The study aims support the mission and priority areas of the NIDDK and are expected to have direct clinical implications for individuals with KS.