C1 esterase inhibitor [human] is a serine proteinase inhibitor derived from human plasma that is used in treating hereditary angioedema. Breastmilk levels of C1 esterase inhibitor have not been measured after exogenous administration in humans. Because of its large molecular weight, amounts in milk are expected to be small. Any C1 esterase inhibitor in breastmilk is probably destroyed in the infant's gastrointestinal tract and not absorbed, except perhaps in neonates. Various international consensus panels state that human plasma-derived C1 esterase inhibitor is considered to be the therapy of choice for both treatment and short- and long-term prophylaxis of maternal hereditary angioedema during lactation. Recombinant human C1 esterase inhibitor is a valid alternative.[1-5] A patient with Factor XII hereditary angioedema was also successfully treated with subcutaneous C1 esterase inhibitor during pregnancy and lactation.[6]