Background: The diagnosis of intellectual disability in children can produce complex grief-related feelings in their parents. Previous studies have focused on the moment of the diagnosis or the early life of the children, and little research has been conducted on their feelings of grief in adulthood. The objective was to analyse the process of grief/loss in parents of adult offspring with intellectual disability. Method: The intentional sampling included sixteen parents who responded to semi-structured interviews. A qualitative study was conducted based on grounded theory. Results: Five categories were identified: "Reception of the diagnosis", "Emotional bonds with the child", "Experience of loss and feelings in response to intellectual disability diagnosis", "Recurrent grief" and "Coping strategies". Conclusions: Recurring feelings of loss experienced by parents in relation to their child's diagnosis persist over time. Specialized emotional interventions are needed to help parents to reduce the intensity of their feelings of grief.