Objectives: To assess quality of life and psychologic adjustment in children and adolescents with early treated phenylketonuria (PKU). Study design: Cross-sectional study. Methods: Assessment of 37 patients with PKU between 3 and 18 years of age (mean, 10.9 years), with parents used as informants. The TNO-AZL Questionnaire for Children's Health-Related Quality of Life and Child Behavior Checklist scores were compared with healthy reference groups. Predictive values of sociodemographic and clinical variables for quality of life and psychologic adjustment were assessed. Results: Most dimensions of quality of life in children with PKU were not different from reference values. The only deviation in the PKU group was a reduction of positive emotions. Psychologic adjustment in patients with PKU was better than in a healthy reference group. Whereas sociodemographic variables and phenylalanine levels in the 12 months preceding the study were not associated to quality of life and psychologic adjustment, a long-term negative effect of higher phenylalanine levels during the first year of life on some dimensions of quality of life and psychologic adjustment could be identified. Conclusions: Although the results may not necessarily apply to patients with PKU in other geographic or socioeconomic contexts, they demonstrate that normal health-related quality of life and good psychologic adjustment is an achievable goal in children with PKU. (J Pediatr 2002;140:516-21)