Phenylketonuria (PKU; OMIM 261600) is an autosomal recessive disorder of phenylalanine metabolism caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH; EC 1.14.16.1). Cognitive problems, neuropsychological abnormalities and psychosocial problems have been reported frequently in children and adolescents with PKU, even in those who are treated early and continuously. However, the developmental consequences in adulthood of growing up with PKU are not well known. The aim of this study was to assess the course of life, sociodemographic outcomes and health-related quality of life in young adult patients with PKU identified on neonatal screening who were continuously on treatment. A total of 32 PKU patients 18 to 30 years old completed the Course of Life questionnaire, the RAND-36 Health Survey, and the cognitive scale of the TNO-AZL Adult Quality of Life (TAAQoL) questionnaire. The results of the Course of Life and Health-Related Quality of Life questionnaires were comparable to controls, except that a higher percentage received special education in primary school. Their educational attainment, however, was comparable to that of their peers. The results of this study demonstrate that although PKU is a chronic disease with the burden of strict dietary control, early and continuously treated patients with PKU can have a normal health-related quality of life and course of life.