Newborn screening for phenylketonuria (PKU) is now the standard of practice. Initial phenylalanine blood levels of 240 mumol/L result in referral of affected newborns to medical facilities experienced in caring for patients with metabolic disorders. This case report concerns a female infant born in 1976 with a presumptive positive PKU screening test on the third day of life of 240 mumol/L phenylalanine. Follow-up levels while the mother was breast feeding on the sixth day of life were 324 and, on the 27th day, 312 mumol/L. She was subsequently lost to follow-up at age 11 years, but returned at 19 years of age due to pregnancy, with a blood phenylalanine level of 132 mumol/L. Mutation studies then were performed documenting that she was a carrier for the phenylalanine hydroxylase gene and did not have hyperphenylalaninemia. The mother's parents and the infant were also genotyped confirming heterozygosity. The infant on follow-up is completely normal, following a normal pregnancy.