Background: Multiple Acyl-CoA Dehydrogenase Deficiency (MADD) is a rare autosomal recessively inherited disorder of fatty acid metabolism due to ETFA, ETFB or ETFDH mutations. Molecular defects in the ETFDH gene were found to be responsible for a specific sub-group of patients affected with a variant of the disease responsive to riboflavin. Methods: We report on a man of 69 years complaining of a progressive proximal muscle weakness to the four limbs, associated with muscle pain and elevated CPK (1751 U/L). Results: Histological and biochemical results were strongly suggestive for a defect of fatty acids oxidation. Muscle biopsy showed a lipid storage myopathy. Urinary organic acid analysis showed a marked increase of 2-hydroxyglutaric and ethylmalonic acids excretion and the acylcarnitine profile demonstrated a significant increase of long and medium-chain carnitine esters. An oral therapy of L-carnitine and riboflavin supplementation dramatically improved the clinical picture. Sequence analysis of ETFDH gene showed the presence of a single nucleotide insertion in heterozygous form (c.936insA; p.N313Kfs314X). This insertion causes a frameshift leading to the formation of a truncated protein. Conclusions: Our patient presents clinical, histological and biochemical findings suggestive of MADD even if we failed to find the second causative mutation in ETFDH gene. Moreover, we excluded a copy number gene variation, by probing all the ETFDH exons. We can assume that causative mutations for this gene can act either as a loss of functional protein, when transmitted in autosomal recessive condition or as an aploinsufficient protein when present in one allele only.