Borderline personality disorder (BPD) is a serious mental illness. It causes a person's moods, relationships, self-image, and behavior to be unstable from one day to the next. This can hurt family and work life, the ability to make long-term plans, and the person's sense of self-identity. Women are more likely than men to have BPD. Researchers are still learning about BPD, its causes, and its symptoms. There are treatments for BPD.