This article explores the meaning of diagnostic tests for people with chronic back pain. Lower back pain is one of the most common health problems in the US. Five to ten percent of the patients who visit a primary care provider for back pain ultimately develop a chronic condition. We draw on interviews with chronic back pain patients in Atlanta, Dallas and Seattle to argue that testing constitutes an important element in the legitimation of pain for these patients. We discuss three aspects that make testing an area of concern for patients: a strong historical connection between visual images and the medicalization of the interior of the body, a set of cultural assumptions that make seeing into the body central to confirming and normalizing patients' symptoms, and the concreteness of diagnostic images themselves. Our interviews show that when physicians cannot locate the problem or express doubt about the possibility of a solution, patients feel that their pain is disconfirmed. Faced with the disjunction between the cultural model of the visible body and the private experience of pain, patients are alienated not only from individual physicians but from an important aspect of the symbolic world of medicine. This paper concludes by suggesting that a fluid, less localized understanding of pain could provide a greater sense of legitimacy for back pain patients.