Background: This study assessed the co-occurrence of cognitive problems and difficult temperament characteristics in children aged 3 to 5 years exhibiting aggressive behavior. Methods: Thirty-one children with high ratings on the Aggressive Behavior subscale of the Achenbach Child Behavior Checklist or Teacher Report Form were recruited from a school district-based clinic. Cognitive ability and temperament were assessed with the Differential Ability Scales and the Carey Temperament Scales Behavioral Styles Questionnaire, respectively. Cluster analysis was applied to these variables. Results: Three prominent groups of children sharing cognitive and temperament characteristics emerged. The first, comprising 41.9% of the sample, demonstrated low adaptability, low persistence, high activity, negative mood, and low rhythmicity, along with borderline-deficient cognitive scores. The second (38.7%) demonstrated low adaptability, low persistence, high activity, and negative mood, along with average cognitive scores. The third (19.4%) demonstrated essentially midrange temperament characteristics and average cognitive scores. All 3 groups shared temperamental tendencies toward a higher threshold of response. Most children in the sample also had significant sleep disturbances (54.5%), other health problems (63.6%), and/or suspected developmental problems (54.5%). Conclusions: The widespread presence of sleep, health, cognitive, temperament, and developmental problems in this sample supports the involvement of medical and developmental health experts in teams evaluating young children who present with high levels of aggressive behavior. The diverse nature of cognitive-temperament profiles in this sample also suggests a need to assess cognitive ability and temperament in young, aggressive children to provide individualized interventions. (Contains 6 tables.)