The current study discussed the development and validation of the Checklist for Differential Diagnosis of Attentional Problems (CDDAP), a tool for use with adults seeking diagnosis and treatment of an Attention Deficit Hyperactivity Disorder. Normative data are provided on three subject groups (ADHD adults, controls, and adults with other psychiatric disorders). Convergent validity was established with the SCL-90, and criterion validity established through comparing scaled scores with final diagnoses. Overall, this measure was accurate at differentiating adults with ADHD from controls and adults with other psychiatric disorders. Results indicated that the CDDAP was also able to identify other psychiatric disorders with 71 to 92% accuracy, depending on the disorder.