Phenylketonuria is the most common inborn error of amino acid metabolism. The defect is due to mutations in genes encoding enzymatic proteins, which result in deficient or impaired activity of phenylalanine hydroxylase, an enzyme responsible for converting phenylalanine into tyrosine. The therapy is based on an individually selected and appropriately balanced normal-protein, low-phenylalanine diet. The aim of the study was to identify and evaluate the most common difficulties in the maintenance of strict diet and to assess the objective measurable parameters indicating compliance with the low-phenylalanine diet. The study included 63 patients with phenylketonuria aged between 3 months and 44 years (mean age 12.65 ± 10.41 years), attending the Outpatient Metabolic Clinic in the Polish Mother’s Memorial Hospital Research Institute in Lodz. All patients were asked to complete an original questionnaire. The analyses showed that high costs of a lowprotein diet, poor taste of phenylalanine-free dietary supplements as well as reduced appetite due to the use of restrictive diet are the major problems faced by patients. A total of 68% of adult and 16% of adolescent patients failed to comply with the diet, as confirmed by increased serum phenylalanine levels. Young patients and their parents are more aware of the demand for phenylalanine than adult patients.