Many young women who were diagnosed as having phenylketonuria (PKU) during routine neonatal screening and effectively treated during childhood are now of childbearing age. Recent reports suggest that maternal dietary therapy instituted before conception may improve the likelihood of a successful pregnancy and normal offspring. However, it is not known whether the intake of phenylalanine (phe) should be restricted during lactation. While phe levels in breast milk from women with PKU are markedly elevated, to our knowledge serum phe levels have not been measured in nursing newborn infants of PKU mothers. The present case report describes the pregnancy and early lactation of a mother with PKU, including serial measurements of serum phe levels in her offspring while being breast-fed.