Ubiquinone (coenzyme Q(10) or CoQ(10)) is a lipid-soluble component of virtually all cell membranes and has multiple metabolic functions. Deficiency of CoQ(10) (MIM 607426) has been associated with five different clinical presentations that suggest genetic heterogeneity, which may be related to the multiple steps in CoQ(10) biosynthesis. Patients with all forms of CoQ(10) deficiency have shown clinical improvements after initiating oral CoQ(10) supplementation. Thus, early diagnosis is of critical importance in the management of these patients. This year, the first molecular defect causing the infantile form of primary human CoQ(10) deficiency has been reported. The availability of genetic testing will allow for a better understanding of the pathogenesis of this disease and early initiation of therapy (even presymptomatically in siblings of patients) in this otherwise life-threatening infantile encephalomyopathy.