This longitudinal study examined the relation between the instability of maternal intimate relationships and the school behavior of 3rd-grade children from economically disadvantaged families. After ecological correlates were controlled, chronic relationship instability predicted externalizing behavior for boys and girls and internalizing behavior for girls, but not academic competence. In addition, past and recent instability had independent effects: Recent instability moderated the relation for past instability, and child adjustment in highly unstable families varied with verbal ability and aspects of the family ecology. The theoretical implications concern conceptualizations of the diverse and dynamic nature of family arrangements experienced by disadvantaged children.