Background: The support of relatives during older patients’ fast-track treatment programmes is important; however, knowledge is needed to strengthen the involvement of the relatives in fast-track treatment programmes. Purpose: To generate a substantive grounded theory of relatives’ behaviour patterns in older patients’ fast-track treatment programmes during total hip or knee replacements. Methods: Grounded theory based on Glaser’s systematic generation of theory was used. Data was collected from 2010 to 2011 in orthopaedic wards at two Danish university hospitals through 14 non-participant observations, 14 post-observational interviews, and five interviews with seven relatives of patients over the age of 70. Findings: Maintaining Unity emerged as the relatives’ pattern of behaviour through which they prevented the patients from feeling alone through three interchangeable behavioural modes: Protecting Mode, Substituting Mode, and Adapting Mode. Conclusion: The substantive theory of Maintaining Unity offers knowledge of relatives’ desire to provide loving support for the older patients during fast-track treatment programmes.