Loss of function mutations in COQ4 lead to fatal neonatal mitochondrial disorders with CoQ10 deficiency. In two siblings with childhood-onset, slowly progressive ataxia from a consanguineous mating from Turkey, whole exome sequencing identified homozygous missense mutations in COQ4 (Gly55Val). Blood levels of CoQ10 were either below or at the low end of the normal range. The more severely affected of the siblings was given a high dose of CoQ10 (2000 mg/day) for one month, following which significant improvement in neurological signs and symptoms was noted. Our report indicates that COQ4 mutations are a rare cause of ataxia and that CoQ10 supplementation is a personalized treatment for this form of ataxia.