The findings show that a father's positive interaction with a child early in his/her life creates a social environment conducive to a child's good mental health in middle childhood. On the other hand, children who experienced low interaction with their fathers in infancy developed behavioral, autonomic, and adrenocortical responses that became risk factors for adverse mental health symptoms in middle childhood. The highest symptom-severity scores were for children with high autonomic reactivity, low levels of early interaction with their fathers, and mothers with symptoms of depression. The findings emphasize the importance of having both parents involved in the social development of their children in order to reduce any risk for mental disorders posed by the child's biological characteristics. The study did not determine which specific features of a father's early interaction with a child acted to reduce the risk for mental disorders in middle childhood. A total of 120 children and their parents and teachers were recruited from the Wisconsin Study of Families and Work (WSFW) in 1998, when the children were 7 years old. Fathers' involvement with their children was assessed with two scales completed when the children were 1 year old. On the Division of Childcare Scale, fathers reported on 13 items related to the father's frequency and characteristics of interaction with the child. Children's behavioral, autonomic, and adrenocortical reactivity were measured by standardized indicators of biobehavioral sensitivity to social conditions during a 4-hour assessment in the home in 1998. Mental health symptoms were assessed at 9 years old, using parent, child, and teacher reports. 2 tables, 3 figures, and 68 references