Objective: International appeals call for interventions to prevent aggression and other behavioral problems in individuals with dementia (IWD). Aggression Prevention Training (APT), based on intervening in three contributors to development of aggression (IWD pain, IWD depression, and caregiver–IWD relationship problems) aims to reduce incidence of aggression in IWD over 1 year. Design: Randomized, controlled trial. Setting: Three clinics that assess, diagnose, and treat dementia. Participants: Two hundred twenty-eight caregiver–IWD dyads who screened positive for IWD pain, IWD depression, or caregiver–IWD relationship problems randomized to APT or Enhanced Usual Primary Care (EU-PC). Intervention: APT, a skills-based intervention delivered over 3 months to address pain/depression/caregiver–IWD relationship issues. EU-PC included printed material on dementia and community resources; and eight brief, weekly support calls. Measurements: The primary outcome was incidence of aggression over 1 year, determined by the Cohen Mansfield Agitation Inventory-Aggression Subscale. Secondary outcomes included pain, depression, caregiver–IWD relationship, caregiver burden, positive caregiving, behavior problems, and anxiety. Results: Aggression incidence and secondary outcomes did not differ between groups. However, in those screening positive for IWD depression or caregiver–IWD relationship problems, those receiving EU-PC had significant increases in depression and significant decreases in quality of the caregiver–IWD relationship, whereas those receiving APT showed no changes in these outcomes over time. Conclusion: The cost to patients, family, and society of behavioral problems in IWD, along with modest efficacy of most pharmacologic and nonpharmacologic interventions, calls for more study of novel preventive approaches.