The child of a phenylketonuric woman is exposed during pregnancy to a high risk of growth retardation and malformation. The frequency of these abnormalities is proportional to the maternal phenylalanine blood concentrations. If a strict low protein diet is followed before conception and throughout gestation the risks of abnormalities are not higher than in the normal population. The maternal blood phenylalanine levels must be maintained between 120 and 250 mumol/l and the tyrosine blood levels between 45 and 90 mumol/l. Weekly blood analyses are mandatory. Regular dietary controls are necessary to assure that the adequate energetic intake and the correct amounts of vitamins and minerals recommended for a pregnant woman are sustained. A case report is the opportunity to discuss certain practical aspects concerning the monitoring of the pregnancy of a phenylketonuric woman and to make general recommendations.