Deficiency of very-long-chain acyl-CoA dehydrogenase (VLCAD) is an autosomal recessive disease. Most common phenotypes occur in the neonatal period or in childhood with cardiomyopathy, hepatomegaly, and hypoketogenic hypoglycemia. Juvenile/adult-onset is characterized by exercise intolerance and recurrent rhabdomyolysis triggered by prolonged exercise or fasting. This article reports a patient with the homozygous mutation c.1097G>A (p.R366H) in the ACADVL gene. In Portugal, VLCAD deficiency became part of the neonatal screening plan in 2004, and as of 2012, 8 early-onset cases have been diagnosed, giving an incidence rate of 1:97.238 per 737.902 newborns. This patient was diagnosed outside of the neonatal screening plan. Beta-oxidation defects pose a diagnostic challenge because of their transient clinical and laboratorial manifestations and the absence of morphological changes in muscle biopsy further complicate matters, especially in the late-onset forms of the disease. The adult phenotype of VLCAD deficiency is highlighted, emphasizing the need for a high suspicion index and the value of tandem mass spectrometry for the diagnosis.