PKU patients on a relaxed diet may be at risk for micronutrient deficiencies. 
To investigate micronutrient supply in phenylketonuria (PKU) patients on a relaxed diet.  Sixty-seven patients (6-45 years) with a phenylalanine tolerance ≥ 600 mg/day were included in the study.  From a 3-day diet record, protein supply as well as consumption of essential amino acids and several micronutrients were assessed and compared with the current recommendations and data for the healthy population.  Protein supply and consumption of all essential amino acids were sufficient in all patients.  Supply of micronutrients depended on dietary regime.  Patients with a total protein supply of 120% or more of the recommended amount and at least 0.5 g protein per kg body weight from amino-acid mixture (AAM) were sufficiently supplied with all investigated micronutrients.  All patients without AAM supplement showed severe micronutrient deficiencies in their diet records.  PKU patients under a relaxed diet are at risk of an insufficient nutrient supply, if they have first no substitution with AAM, second a protein supply less than 0.5 g per kg body weight from AAM or third a total protein supply less than 120% of the recommendations.  Therefore, close monitoring, specific dietary counseling and potential supplementation is mandatory to prevent micronutrient deficiencies in PKU patients.