Methods of treating or suppressing mitochondrial diseases, such as Friedreich's ataxia (FRDA), Leber's Hereditary Optic Neuropathy (LHON), mitochondrial myopathy, encephalopathy, lactacidosis, and stroke (MELAS), Kearns-Sayre Syndrome (KSS), are disclosed, an well as compounds useful in the methods of the invention, such as 4-(p-quinolyl)-2-hydroxybutanamide derivatives. Methods and compounds useful in treating other disorders such as amyotrophic lateral sclerosis (ALS), Huntington's disease, Parkinson's disease and pervasive developmental disorders such as autism are also disclosed. Energy biomarkers useful in assessing the metabolic state of a subject and the efficacy of treatment are also disclosed. Methods of modulating, normalizing, or enhancing energy biomarkers, as well as compounds useful for such methods, are also disclosed.