Coenzyme Q10 (CoQ10), also known as ubiquinone, is an endogenously synthesized lipid that transfers electrons from complexes I (NADH-ubiquinone oxidoreductase) and II (succinate-ubiquinone oxidoreductase) and from the oxidation of fatty acids and branched-chain amino acids (via flavin-linked dehydrogenases) to complex III (succinate-cytochrome c oxidoreductase) of the mitochondrial respiratory chain. CoQ10 deficiency is an autosomal recessive disorder presenting five main phenotypes; cerebellar ataxia is the most common, and it is characterized by cerebellar ataxia and cerebellar atrophy, variably associated with other manifestations as neuropathy, seizures, cognitive compromise, and muscle weakness. Molecular defects have been described in three different genes: APTX, ADCK3, and ANO10. Early diagnosis is important because cases can benefit from CoQ10 supplementation. Here, we present a 16-year-old man with cerebellar ataxia and upper motor neuron dysfunction, CoQ10 deficiency in muscle and fibroblasts, and compound heterozygous mutations in ADCK3, analog of the yeast CoQ8, involved in the regulation of CoQ biosynthesis.