Person-centered healthcare requires providers to appreciate the knowledge and perspectives of patients. Effective and appropriate care depends on such knowledge. Medical institutions can only function well when they acknowledge patients’ own experiences. Yet a range of evidence shows that professionals and organisations often ignore patients’ own knowledge about their condition and treatment. This article aims to explain why this epistemic injustice occurs and persists. (Epistemic: to do with knowledge. Justice, because professionals and organisations do wrong when they bypass or deny patients’ own knowledge.) The explanation focuses on problems of power and accountability. Illness is a disempowering experience, partly for bodily and psychological reasons, partly because the ill person depends on others for help, partly because professionals and organisations are specially empowered in order that they may help. Occupying a lesser power position, patients often walk a tightrope between conflicting demands and may be caught in double binds: situations where every possibility for action risks bad outcomes. By contrast, professionals need not notice their greater power position and how this opens up paths of least resistance, whereby it is easy to ignore or belittle patients’ knowledge. When it is hard for patients to voice their “complaints” (the details of their illness, their sense of being badly treated), accountability falters. Healthcare providers may see themselves as expert and responsible, even as they fail many persons they are meant to help.