Reported here is a novel presentation of late onset glutaric aciduria type 2 in two Thai siblings. A 9-year-old boy presented with gradual onset of proximal muscle weakness for 6 weeks. The initial diagnosis was postviral myositis, and then polymyositis. Electromyography and nerve conduction velocity testing indicated a myopathic pattern. Muscle biopsy revealed excessive accumulation of fat. Acylcarnitine profiling led to the diagnosis of glutaric aciduria type 2. Immunoblot analysis of electron-transferring-flavoprotein and its dehydrogenase electron-transferring-flavoprotein dehydrogenase led to mutation analysis of the ETFDH gene, which revealed two different pathogenic mutations in both alleles and confirmed the diagnosis of glutaric aciduria type 2 caused by electron-transferring-flavoprotein dehydrogenase deficiency. The boy recovered completely after treatment. Later, his younger sibling became symptomatic; the same diagnosis was confirmed, and treatment was similarly effective. Acylcarnitine profiling was a crucial investigation in making this diagnosis in the presence of normal urine organic acid findings. Late onset glutaric aciduria type 2, a rare cause of muscle weakness in children, should be included in the differential diagnosis of myopathy. Copyright © 2010 Elsevier Inc. All rights reserved.