A prospective cohort study was done to assess the effects of value bias and the inappropriate use of the availability heuristic on physicians' judgments of the probability of bacteremia. Subjects of the study were 227 medical inpatients in a university hospital who had blood cultures done. Estimates of the probabilities that individual patients would have positive blood cultures were collected from the house officers who ordered the cultures. Clinical data and culture results were also obtained. Based on the data the authors calculated "value variables," reflecting doctors' assessments of the risks that individual patients would die in the hospital if they were to have bacteremia. "Recalled experience variables" reflected the doctors' recollections of recent experiences with patients with bacteremia. The physicians significantly overestimated the likelihood of bacteremia for most of their patients. Their ROC curve for this diagnosis showed moderate discriminating ability (area = 0.687, SE = 0.073). Two recalled experience variables were significantly associated with the physicians' probability estimates. The value variables were significantly inversely associated with them. These relationships were independent of several clinical variables and measures of disease severity. The physicians' intuitive diagnostic judgments were thus influenced by the availability heuristic and by wishful thinking, a form of the value bias. The availability heuristic may mislead physicians by causing them to believe that random variations in the prevalence of a nonepidemic disease represent real trends. Wishful thinking may lead physicians to underestimate the likelihood of a disease for patients most at risk for its consequences. Teaching physicians to develop better judgmental strategies may improve the quality of their judgments and hence their patient care.