This study explores how family members define mistreatment in formal old age care. Twenty-one interviewees were recruited from a Swedish association for relatives of care recipients. Argumentation analysis was used and four warrants (normative grounds) was identified in family members’ argumentation about mistreatment. The warrants used were based on physical harm, psychological harm, social deficiencies and identity subversion. Particular interest was devoted to a how family members described elder mistreatment as the violation of an older person’s identity and to the way interviewees claimed that the older person’s identity and habits should be maintained while he or she receives formal care. The warrants used by family members show how difficult it is to objectively assess what elder mistreatment is (and not is) and how to measure it. In relation to staff, some family members take on a role as guardians of dignity. This has implications for how staff choose to understand and react to complaints and suggestions for change. Our analysis indicates that family members’ complaints about matters, which staff sees as “petty details”, can be interpreted as part of a greater project of guarding and preserving the identity and personality of the older person.