Attention deficit/hyperactivity disorder (ADHD) is one of the most common psychiatric disorders in childhood and adolescence. Rating the severity of psychopathology and symptom load is essential in daily clinical practice and in research. The parent and teacher ADHD-Rating Scale (ADHD-RS) includes inattention and hyperactivity/impulsivity subscales and is one of the most frequently used scales in treatment evaluation of children with ADHD. An extended version, mADHD-RS, also includes an oppositional defiant disorder subscale. The partial credit Rasch model, which is based on item response theory, was used to test the psychometric properties of this scale in a sample of 566 Danish school children between 6 and 16 years of age. The results indicated that parents and teachers had different frames of reference when rating symptoms in the mADHD-RS. There was support for the unidimensionality of the three subscales when parent and teacher ratings were analyzed independently. Nonetheless, evidence for differential item functioning was found across gender and age for specific items within each of the subscales. The findings expand existing psychometric information about the mADHD-RS and support its use as a valid and reliable measure of symptom severity when used in age- and gender-stratified materials.