We report on a 21-year-old woman with classic phenylketonuria, who presented at our outpatient clinic at week 14 of pregnancy. Despite intensive preconceptional counselling about the risk of raised Phenylalanine (Phe) levels for the offspring and nutritional consultations about the necessity to be on a Phe-restricted diet she had elevated blood Phe concentrations. Phe level could be lowered to the recommended range during a stay as an inpatient, but the patient was not able to maintain the recommended levels due to non-compliance. The patient delivered a newborn with classic maternal PKU syndrome (microcephaly, brachygnathia, congenital heart defect and psychomotoric retardation), which is nowadays rarely seen under preconceptional Phe-restricted diet. With more PKU patients reaching the childbearing age, intensive preconception counselling about maternal PKU syndrome is of pivotal importance for the women. However, a major factor in preventing Phe embryopathy is patient compliance in keeping the diet, which was insufficient in the case presented.