We report a positive newborn screen for 3-hydroxyisovalerylcarnitine (C5OH) with an absence of 3-methylcrotonyl-coenzyme A carboxylase deficiency in the neonate. Subsequent blood tests demonstrated persistently elevated C5OH. Serial testing of the mother identified markedly elevated C5OH in both maternal blood and breast milk. High C5OH milk concentrations provide a significant source of C5OH to the nursing neonate and possibly explains its persistent elevation in the neonate, a commonly observed finding in maternal 3-MCC deficiency.