The purpose of this study was to compare physicians' perceptions about managed care restrictions on drug prescribing with objective measures of the restrictions' effects. When asked a general question, 17 emergency medicine physicians in one urban, university hospital answered that they had to prescribe an antibiotic that was not their first choice because of managed care restrictions 32% of the time. The actual frequency of prescribing other than first-choice antibiotics, which was determined by asking the same physicians about the prescription of specific antibiotics for specific patients seen recently in the emergency department, was 6% ( p <.0001). We conclude that emergency medicine physicians treating patients in one managed care system significantly overestimated the restrictions imposed by managed care formularies on their antibiotic prescribing practices. Additional studies are warranted to measure the extent of this bias.