Isovaleric acidemia is a rare autosomal recessive inborn error of leucine catabolism caused by deficiency of isovaleryl coenzyme A dehydrogenase. This enzymatic deficiency leads to severe metabolic derangement, manifested clinically as vomiting, dehydration, and acidosis progressing to seizures, coma, and death. The two phenotypic expressions are the acute severe and the chronic intermittent form. The acute severe phenotype typically results in death during early infancy, whereas patients with the chronic intermittent form are asymptomatic at baseline but have episodes of acute metabolic decompensation, usually in the setting of infection, physical exertion, or ingestion of protein-rich food. This case illustrates how inborn errors of metabolism resulting in organic acidemia can be manifested in adults and why the internist needs to be aware of them.