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To investigate the relation between physicians' predicted probabilities of pneumonia and their utilities for ordering chest x-rays to detect pneumonia, the authors studied 52 physicians who ordered chest x-rays of 886 patients presenting to an emergency department with fever or respiratory complaints. Physicians estimated the probability of pneumonia prior to obtaining the results of the chest x-ray. Utilities were assessed by asking physicians to consider a hypothetical patient presenting with acute respiratory symptoms, with unknown chest x-ray status, and to rank on a scale from +50 ("best thing I could do") to -50 ("worst thing I could do") their rating scale utilities for not diagnosing pneumonia and not ordering a chest x-ray when the patient had pneumonia (i.e., missing a pneumonia), and for diagnosing pneumonia and ordering a chest x-ray when the patient did not have pneumonia (i.e., ordering an unnecessary x-ray). The utility for ordering an unnecessary x-ray was negatively correlated with average predicted probability (r = -0.1495, p = 0.29), whereas the utility for missing a pneumonia was positively correlated with average predicted probability (r = 0.2254, p = 0.11), although the correlations were not statistically significant. Relative chagrin, defined as the difference in these utilities, was significantly inversely correlated with average predicted probability (r = -0.2992, p less than 0.035), even after adjusting for the prevalence of pneumonia seen by each physician (partial r = -0.42, p less than 0.0027). It is concluded that physicians who experienced greater regret over missing a pneumonia than over ordering an unnecessary x-ray estimated lower probabilities of pneumonia for patients for whom they ordered x-rays.(ABSTRACT TRUNCATED AT 250 WORDS)