The adverse effects of screening are not commonly studied. False-positive tests lead to discomfort, costs, and risks from additional diagnostic and therapeutic procedures. False-negative tests lead to a sense of security and delays in seeking medical help when symptoms develop. Labeling an individual with a false-positive test, or with a true-positive test for which there is no evidence that intervention makes a difference, e.g., intervention on an 80-year-old asymptomatic woman with hypercholesterolemia, can have a markedly negative impact on the quality of life. Interpreting statistical abnormalities out of clinical context, e.g., lending importance to a multiphasic blood screen showing "high" alkaline phosphatase in a teenager, leads to unnecessary costs and anxiety. The cost of screening programs that may not have been shown to do more good than harm is already having an impact on the resources available to diagnose and treatment symptomatic persons. Premature implementation of unproved screening programs will continue to decrease physician and public confidence in prevention.