Phenylketonuria is a genetic disease affecting 1:10,000 to 14,000 live births. In NSW there is an average of nine cases diagnosed each year (Dietitians Working Party 1996). This paper discusses the management of phenylketonuria, and in particular the value of breastfeeding, complemented with a low phenylalanine infant formula, in facilitating easier maintenance of satisfactory phenylalanine blood levels. The 'diet for life' approach to managing phenylketonuria is to avoid long-term neurological deficits and, in particular, the risk that maternal PKU, which is not under strict dietary control, will have adverse effects on infants born of mothers with the disease. There have been 31 successful pregnancies to 1997 managed by the Nutrition and Dietetics Department of The Children's Hospital at Westmead, Sydney. The Maternal PKU diet is presented with the case of a client with phenylketonuria who has achieved two normal pregnancies and breastfed her second child for six months.