Evidence-based health care has been described as a new paradigm for practicing medicine. 2 From the primary care physician's point of view, however, it might better be described as a formal combination of rigorous data and common sense. The common sense is the recognition that tests are worthwhile if they are reliable enough to change decision making. It is also the recognition that treatments are worthwhile if they lead to a better outcome valued by the patient: longer life, fewer symptoms, and particularly in the case of cancer, reduced fear. The rigorous component is the application of the basic science of clinical medicine, 12 clinical epidemiology. Initially, it may strike the reader as strange to suggest that the primary care physician should bring rigorous methods to the detection, diagnosis, and treatment of malignancies. No other area of medicine is more extensively researched in the hard sciences. Basic physiology, immunology, cell biology, and pharmacology have been and are being explored in great depth and detail, and chemotherapeutic regimens are evaluated in well-designed clinical trials. However, the degree of sophistication that characterizes basic research in cancer does not always extend to decision making in the clinic. The primary care physician often must be the one to ask the critical questions: • Will this test bring us news we can use? • How often is the information wrong? • Is there a treatment that makes patients better? • For whom does a treatment really work, how often does it work, and how well? Clinical epidemiology, as the basic science of primary care practice, provides the basis for framing and answering these questions. Evidence-based health care means deciding with the patient the course of testing or treatment based on real, data-based answers to these questions, rather than on the traditional experts' subjectively-interpreted clinical experience.