In many cases, X-linked conditions are transmitted through families "silently" until the first affected individual is diagnosed. Grandmothers are often then tested to help determine the risk to other family members. To date, psychosocial research on carriers of X-linked conditions has focused primarily on mothers and sisters of affected males. In the wider social science literature, studies on grandparents of children with disabilities have centered on their role within the family and relationship with the grandchild. We therefore know little about the impact of carrier testing for a genetic condition on grandparents. This qualitative study aims to contribute towards filling that gap. This study included thirteen grandmothers in families with Fragile X or Duchenne muscular dystrophy; ten had living affected grandsons and three had daughters who chose not to continue with affected male pregnancies after prenatal diagnosis. All thirteen took part in semi-structured interviews and provided a rich and varied data source for conducting thematic analysis. Most of the grandmothers expressed recurring feelings of guilt and a strong sense of responsibility for what had occurred in the family. Other themes included feelings of shock after receiving their test result, changes in family relationships and searching to make sense of the inheritance within the context of the family's experience. This study provides evidence that X-linked carrier testing can have a profound and lasting impact on grandmothers. Although genetic counseling for X-linked conditions is often focused on the potential reproductive implications for carriers, these findings suggest that grandmothers should also be offered genetic counseling when tests are carried out, because of the likely psychosocial impact of a positive test result.