Objective: To study how early father involvement and children's biobehavioral sensitivity to social contexts interactively predict mental health symptoms in middle childhood. Method: Fathers' involvement in infant care and maternal symptoms of depression were prospectively ascertained in a community-based study of child health and development in Madison and Milwaukee, WI. In a subsample of 120 children, behavioral, autonomic, and adrenocortical reactivity to standardized challenges were measured as indicators of biobehavioral sensitivity to social context during a 4-hour home assessment in 1998, when the children were 7 years of age. Mental health symptoms were evaluated at age 9 years using parent, child, and teacher reports. Results: Early father involvement and children's biobehavioral sensitivity to context significantly and interactively predicted symptom severity. Among children experiencing low father involvement in infancy, behavioral, autonomic, and adrenocortical reactivity became risk factors for later mental health symptoms. The highest symptom severity scores were found for children with high autonomic reactivity that, as infants, had experienced low father involvement and mothers with symptoms of depression. Conclusions: Among children experiencing minimal paternal caretaking in infancy, heightened biobehavioral sensitivity to social contexts may be an important predisposing factor for the emergence of mental health symptoms in middle childhood. Such predispositions may be exacerbated by the presence of maternal depression. (Contains 3 figures and 2 tables.)