We aimed to determine whether it is possible to discriminate between children with attention deficit hyperactivity disorder (ADHD) and children with reactive attachment disorder (RAD) using standardized assessment tools for RAD. The study involved 107 children: 38 with a diagnosis of RAD and 30 with ADHD were recruited through community child and adolescent mental health services (CAMHS) and specialist ADHD clinics. In addition, 39 typically developing children were recruited through family practice. Clinicians were trained to use a standardized assessment package for RAD using a DVD with brief follow-up support. Discriminant function analysis was used to identify the items in the standardized assessment package that best discriminated between children with ADHD and children with RAD. Clinicians' ratings of RAD symptoms were reliable, particularly when focusing on eight core DSM-IV symptoms of RAD. Certain parent-report symptoms were highly discriminatory between children with ADHD and children with RAD. These symptoms included "cuddliness with strangers" and "comfort-seeking with strangers". A semi-structured interview with parents, observation of the child in the waiting room and teacher report of RAD symptoms aided diagnostic discrimination between the groups. Clinical diagnosis of RAD can be made reliably by clinicians, especially when focusing on eight core RAD symptoms. Clear discrimination can be made between children with RAD and children with ADHD.