Involvement of grandparents in grandchildren's lives is important for grandchild well-being. Studies suggest that the quality of relationships between grandparents and their adult children may "spill over" to the quality of their relationships with their grandchildren. However, no research has tested whether grandparent alcohol use disorder (AUD) disrupts intergenerational relationships. This is important because grandchildren may not benefit from closeness with grandparents with AUD. In a sample of 295 parents and their children (N = 604) from a larger longitudinal study oversampled for familial AUD, this study tested whether grandparents (G1, "Generation 1") with AUD had poorer relationships with their adult children (G2 "Generation 2") in terms of greater stress and less support provided and less closeness with their grandchildren (G3, "Generation 3"). We also tested whether poorer G1-G2 relationship quality predicted less G1-G3 closeness. Finally, we tested whether effects of G1 AUD on G1-G3 closeness were explained by G1-G2 relationship quality. Separate models were estimated for maternal and paternal grandparents. We found evidence for three indirect effects. First, G1 maternal grandparent AUD predicted greater stress in the G1 grandmother-G2 mother relationship, which was associated with greater closeness between maternal grandmothers and grandchildren. This indirect effect was replicated in G1 paternal grandfathers and G2 fathers. Additionally, G1 paternal grandparent AUD was associated with lower levels of support provided from G1 grandfathers to G2 fathers, which predicted less closeness between paternal grandfathers and grandchildren. The results demonstrate complex intergenerational effects of AUD on family relationships and consistent with the hypothesized "spillover" effect of intergenerational relationships. (PsycInfo Database Record (c) 2023 APA, all rights reserved).