Medical care in the United States continues to face tremendous financial pressures. Public and private health policy claim to encourage primary care and preventive services, but also discourage services that have not been demonstrated to be effective and/or cost-effective. This article suggests a model to illustrate the conceptual relationship between traditional American medical care and "evidenced-based" medicine. It further examines how the lack of an adequate research base makes a move to purely evidence-based care premature for primary care and prevention services. The paper defines a new conceptual statistic, the uncertainty index, as the proportion of non-refuted current practice that is also not corroborated by research evidence. The greater the uncertainty index, the less appropriate is a clinical model restricted to evidence-based care. Specific theoretical barriers to outcomes research in prevention are discussed and simple criteria to determine the desirable components of care are suggested. The need for theoretical and empirical research into primary care and prevention, especially for children, is emphasized. Care that is of low risk, not of extremely high cost, and that is generally believed useful by the community of practitioners is particularly desirable in the absence of data refuting its value.