Introduction:Glutaric acidemia type II (GA II) typically presents with profound metabolic acidosis, hypoketotic hypoglycemia, and mild dysmorphic features in newborns. Since its description, 11 patients with adult‐onset GA II have been reported.Case Report:A 23‐year‐old woman presented at 6 months gestation with preterm labor and weakness for 3 months. Her weakness worsened to the point of not being able to walk despite treatment with corticosteroids for presumed myositis. Biopsies of skeletal muscle revealed extensive lipid myopathy. Metabolic studies and urine organic acids were suggestive of a multiple acyl‐CoA dehydrogenase deficiency. Immunoblot of skin fibroblasts demonstrated a deficiency of electron transfer flavoprotein‐ubiquinone oxidoreductase. Treatment with carnitine, riboflavin, and diet restrictions improved muscle strength and decreased urine organic acids.Conclusion:This patient had the unique presentation of GA II with only myopathy. In adults with late‐onset weakness, metabolic disorders should be considered in the differential diagnosis.