At a consensus conference of key stakeholder communities, participants agreed that IA is a substantial public health and clinical concern. As such, it is a key therapeutic target across multiple disorders and can be measured with sufficient precision to warrant pharmacological studies. Given the current state of knowledge, conference participants agreed that IA should be studied primarily within well-defined diagnostic groups, such as attention deficit/hyper-activity disorder, autism, and bipolar disorder. Although a gold standard for measuring IA does not exist and assessment methods require further refinement, the conference consensus was that current assessment instruments are sufficiently valid and reliable to conduct IA treatment studies; however, additional studies of valid and reliable IA measures are needed. Further, clinicians and researchers should continue to explore and refine distinctions between IA and proactive/planned aggression and the application of these distinctions in clinical, research, and medication-development programs. Conference participants included academic clinicians, researchers, practicing clinicians, representatives of the U.S. Food and Drug Administration and the National Institute of Mental Health, industry sponsors, and patient and family advocates. After evaluating summary presentations on current research evidence, participants were assigned to three workgroups, examined core issues, and generated consensus guidelines in their areas. Workgroup recommendations were discussed by the whole group in order to reach consensus. The conference was held November 4 and 5, 2004. 2 figures, 2 tables, and 65 references