This within-family, longitudinal study including biological and stepfamilies investigated mutual influences between marital conflict and children's behavior problems. Children (4 to 17 years; N=296) residing in 127 families drawn from a general population study were investigated at Time 1 and again 2 years later. These nested data were analyzed using multilevel modeling, controlling for previous child behavior or marital conflict. Marital conflict about children predicted change in children's behavior. Children's behavior also predicted an increase in marital conflict, particularly in stepfamilies. Differences between siblings in exposure to conflict and the extent to which siblings were a source of argument increased more in stepfamilies than in biological families. Boys were exposed to more conflict over time than were girls.