Anxiety disorders are among the most prevalent psychiatric disturbances in childhood. Nonetheless, they often go unrecognized and untreated, which puts the child at risk for developing additional difficulties, such as academic difficulties, depression, and substance abuse. Further knowledge and valid assessment tools are essential to identify at-risk children. The present study investigates (i) the factor structure of the Penn State Worry Questionnaire for Children (PSWQ-C) using a large Danish community sample (N¿=¿933), and (ii) its treatment sensitivity in clinically anxious children (N¿=¿30) treated with cognitive behavioral therapy. Results from the community sample replicated previous findings supporting the strong psychometric properties of the PSWQ-C, and yielded Danish norms and clinical cut-offs for the measure. Clinically anxious children with a generalized anxiety disorder (GAD; N¿=¿10) diagnosis reported significantly higher levels of worry than anxious children without GAD (N¿=¿20). At post treatment, worry levels in children with GAD but not in anxious children without GAD were normalized. Findings regarding worry in the community sample are discussed in light of normal child development. Implications for the use of the PSWQ-C as a useful and important tool in clinical assessment by psychiatrists and psychologists in their treatment of anxious children and adolescents are also discussed.