Mothers of medically at-risk infants were randomly assigned to a Healthy Start intervention (HV) or a cognitive reframing intervention (HV+). Outcome measures were taken at the conclusion of the intervention (1 year) and at the 3-year follow-up visit. At age 3, children in the HV+ condition (in comparison with those in the HV condition) showed fewer aggression problems (as measured by the Child Behavior Checklist). Maternal emotional unavailability (as measured by combined scores on the Beck Depression Inventory and avoidance items on the Conflict Tactics Scale) at the 1-year visit mediated the effects of the intervention on children's aggression at age 3. Findings suggest that an early, cognitively based intervention may lead to reduced child aggression as a result of increased maternal social-emotional availability within the caregiving relationship. (Contains 3 tables and 1 figure.)