A 22-year-old man without pre-existing medical conditions presented to our hospital with a progressive reduction of his physical overall performance, muscle weakness of the extremities, and diarrhea for the last 2 months concomitant with elevated liver enzymes and creatine kinase activity. After ruling out infectious diseases, neoplasia, and autoimmune disorders as a cause of these symptoms, the histology of liver and muscle samples led us to suspect a diagnosis of a rare lipid metabolism disorder. Molecular biologic testing provided the diagnosis of multiple acyl-coA dehydrogenase deficiency with ubiquinone deficiency and late onset. The course of disease was complicated by liver failure and severe pneumonia requiring ventilatory assistance. With the substitution of riboflavin and ubiquinone, the patient showed a gradual recovery of his clinical presentation and an improvement of his laboratory tests. A congenital lipid metabolic disorder might be a rare cause of severe myopathy and hepatopathy in a young adult.