Background Research addressing the experiences of families of adults with Down syndrome and Alzheimer’ s disease in seeking diagnosis and gaining support is limited. The aim of this study was to gain a greater understanding of these processes by exploring the experiences of families and carers in supporting people with Down syndrome and Alzheimer’s disease who had lived most or all of their lives with family. Method Three detailed case studies were created from multiple data sources, and then analysed thematically. Results Families of adults with Down syndrome experienced stress and confusion as they negotiated a service system poorly equipped to meet their needs and professionals more focused on longstanding disability than the recent diagnosis of Alzheimer’s disease. Such overshadowing led to mismanagement by services. Conclusions This research advances understandings of the support needs of people with Down syndrome and Alzheimer’s disease and their families. It exposes gaps in the service system.