Ten physicians recorded their treatment decisions and estimated probabilities of streptococcal infection for patients with sore throats. Of 308 throat cultures, 15 (4.9%) were positive for group A streptococci. The physicians overestimated the probability of a positive culture for 81% of their patients and their estimates and treatment decisions were strongly associated. Of 104 patients treated before culture results were available, only eight had positive cultures. Probability overestimation may have been due to neglect of the low culture-positive rate, assignment of undue importance to weakly predictive or highly intercorrelated clinical features, and a value-induced bias, occurring when features important for treatment are erroneously linked to the likelihood of disease. Cognitive limitations in information processing may limit the effectiveness of pharyngitis management protocols that require subjective estimates of disease probability.