The first wave of practical products from the international effort to produce systematic maps of the human genome and improve DNA sequencing technologies is taking the form of new tools to predict the risk of specific diseases in individual patients. DNA-based tests for molecular mutations associated with clinical syndromes increasingly allow clinicians to detect disease processes and health risks before clinical problems occur, sometimes making prevention possible. The increasing predictive power of medical diagnostics, however, also poses serious health policy challenges at both the professional and societal levels. At the professional level, DNA-based health risk assessments challenge traditional ethical commitments to confidentially, informed consent, and nondirective genetic counseling. At the societal level, these tests challenge institutions and governments to clarify their policies regarding access to opportunities by defining fair uses of genetic health risk information about individuals outside the clinical setting. Underlying all these issues are basic questions about how we interpret the meaning of DNA-based diagnostic findings.