This study investigated the bidirectional effects of change in maladaptive behaviors among adolescents and adults with fragile X syndrome (FXS) and change in their intergenerational family relationships over a 7.5-year period. Indicators of the intergenerational family relationship between premutation carrier mothers and their adolescent or adult son/daughter with FXS included a measure of the quality of the relationship, as well as descriptions provided by mothers of their relationship with their son/daughter (positive remarks, critical remarks). Maladaptive behaviors decreased, maternal positive remarks increased, and maternal critical remarks and relationship quality remained stable over time. Bidirectional effects of change were observed in predicting maladaptive behaviors and maternal positive remarks, although maladaptive behaviors more strongly predicted positive remarks than the reciprocal association. This research suggests prioritizing maladaptive behaviors in the context of family interventions.