Hydrocodone is a narcotic that is widely used, often in nursing mothers. Although case reports suggest that hydrocodone in breast milk sometimes may be problematic for the breastfed infant, no reports exist on the amount of its excretion into breast milk. Two mothers who were taking an acetaminophen and hydrocodone combination product donated pumped milk for analysis of hydrocodone. Their infants received an estimated 3.1% and 3.7% of the maternal weight-adjusted dosage, but the absolute hydrocodone dosages were 8.58 microg/kg per day and 3.07 microg/kg per day because of the differences in the dosages ingested by their mothers. Moderate dosages of hydrocodone appear acceptable during breastfeeding, but more data are needed to determine the maximum safe dosage for nursing mothers. Neonates and preterm infants may be more susceptible than older infants to adverse effects of hydrocodone and its metabolites in breast milk.