Co-Enzyme Q10 Deficiency – an autosomal recessive condition with a clinical spectrum that encompasses at least five major phenotypes: (1) encephalomyopathy characterized by the triad of recurrent myoglobinuria, brain involvement and ragged red fibers; (2) severe infantile multisystemic disease; (3) cerebellar ataxia; (4) Leigh syndrome with growth retardation, ataxia and deafness; and (5) isolated myopathy. The variability of phenotypes suggests genetic heterogeneity, which may be related to the multiple steps in CoQ10 biosynthesis. Clinical presentation includes generalized weakness, exercise intolerance, and recurrent myoglobinuria, proximal muscle weakness, seizures, cognitive impairment, cerebellar symptoms, infantile encephalopathy with renal involvement, retinitis pigmentosa, optic nerve atrophy, bilateral sensorineuronal deafness, nephrotic syndrome, progressive ataxia, cardiomyopathy, hypothermia, lactic acidosis, cerebral and cerebellar atrophy, and developmental delay were associated with renal tubulopathy and ventricular hypertrophy. The clinical presentation of the variant isolated myopathy, recently described in four patients, is subacute onset of exercise intolerance and proximal limb weakness at variable ages. Treatment includes high dose oral CoQ10 supplementation.