The Anxiety Severity Interview for Children and Adolescents (ASICA) was developed for the repeated assessment of the impact of anxiety and control over anxiety symptoms. The ASICA incorporates three main components of anxiety: physical response, avoidant behaviour and anxious thoughts. The objective of this study was to evaluate the psychometric properties of the ASICA in children with anxiety disorder (n = 139, age 8-18 years) and a non-anxious control group (n = 40, age 8-18 years). A confirmatory factor analysis confirmed the intended factor structure. Internal reliability was moderate to good; inter-rater reliability was excellent. Four-week test-retest reliability was good. The ASICA discriminated between anxious and non-anxious children and appeared sensitive to treatment change. A cut-off score of 13 was determined. Convergent validity with anxiety symptoms was moderate; discriminant validity with depressive symptoms was less strong. The results suggest that the ASICA is a reliable instrument that could be used in clinical practice to repeatedly monitor anxiety severity.