Multiple acyl-CoA dehydrogenase deficiency (MADD) is a rare inborn metabolic myopathy affecting fat and protein metabolism. Patients with late-onset MADD typically present with exercise intolerance and muscle weakness. We present a patient with an acute, very late-onset symptom debut at 52 years of age. Over 5 months, the patient deteriorated from asymptomatic to almost complete loss of ambulation. He had a substantial weight loss, head-drop, progressive proximal limb and chewing weakness. Due to the rapid progression, amyotrophic lateral sclerosis, myositis, myasthenia gravis and a paraneoplastic syndrome in relation to underlying malignancy were considered first. A 2-[