According to some claims, molecular markers are set to revolutionize the process of evaluating prognosis and diagnosis for cancer. Research about cancer markers has, however, been characterized by inflated expectations, followed by disappointment when original results can not be reproduced. Even now, disappointment might be expected, in part because rules of evidence to assess the validity of studies about diagnosis and prognosis are both underdeveloped and not routinely applied. What challenges are involved in assessing studies and how might problems be avoided so as to realize the full potential of this emerging technology?