The authors sought to explain regional differences in physicians' accuracies in diagnosing pneumonia by prospectively studying emergency department patients at three sites and analyzing differences in physicians' diagnostic strategies and patient characteristics. They enrolled 1,119 Illinois patients, 150 Nebraska patients, and 142 Virginia patients presenting with fever or respiratory symptoms for whom physicians ordered a chest radiograph because of suspicion of pneumonia. Emergency department physicians recorded patients' clinical findings and estimated the probability that a chest radiograph would show pneumonia. A measure of accuracy, the correlation between physicians' probability estimates and actual outcomes, was 0.41 (95% CI 0.36-0.46) at Illinois, 0.66 (95% CI 0.54-0.75) at Nebraska, and 0.55 (95% CI 0.42-0.65) at Virginia. Physicians' strategies at the three sites differed markedly in their weightings of asthma, signs of consolidation, cough, tachypnea, age, and gender. These differences in weighting paralleled differences in the optimal clinical strategies derived from patient data at the three sites. Differences in diagnostic accuracy were best explained by differences in the difficulties of diagnosing pneumonia in the populations. Physicians at each site used clinical findings in a way that was close to optimal for their location. This type of analysis provides a new tool for understanding the sources of regional variations in clinical practice.