Primary phenylalanine hydroxylase deficiency, also known as phenylketonuria, results in accumulation of phenylalanine in the blood. Early identification and treatment prevents the majority of clinical sequelae to the disease, but psychological and neurodevelopmental problems can occur in some patients. This article reviews the symptoms, diagnosis, classification and strategies of treatment and management of phenylketonuria. Finally we review new pharmacological and non-pharmaco-logical means of treatment.