Attention Deficit/Hyperactivity Disorder (ADHD) and personality traits are relatively stable from childhood and across life span. The purpose of this study was to identify differential and discriminative personality traits between clinical subtypes of ADHD in adults. The Zuckerman-Kuhlman Personality Questionnaire (ZKPQ) and the Millon Multiaxial Clinical Inventory-II (MCMI-II) were administered to a sample of 146 adults with ADHD. Activity and Aggression-Hostility dimensions from the ZKPQ allowed us to properly classify 75.8% of the inattentive and combined subtypes. Data indicates that ADHD is not a homogeneous entity, but rather, there are significant differences in personality characteristics among clinical subtypes. The results have theoretical implications about the connection between ADHD and personality, and clinical implications regarding diagnosis and treatment designs better tailored to the characteristics of each subtype.