Objective: To examine the factor structure of the Multidimensional Anxiety Scale for Children (MASC) with Australian adolescents with and without ADHD. Method: The MASC was administered to 210 high school–aged adolescents (109 males, 101 females), 115 of whom were clinically diagnosed as ADHD (86 males, 29 females). The remaining 95 were non-ADHD community comparisons. Results: Analyses supported a three-factor model, with a reduced item pool, which combined the Harm Avoidance and Separation Anxiety scales together. This model was invariant across younger and older participants, and across boys and girls. The model was largely invariant across ADHD and non-ADHD groups. The ADHD group had significantly higher Physical Symptom factor scores than the non-ADHD group. Conclusion: The MASC is useful for assessing anxiety in adolescents with and without ADHD, but items reflecting the Harm Avoidance and Separation Anxiety scales may need revising. (J. of Att. Dis. XXXX; XX(X) X-XX)