Prescribing antibiotics to patients with a history of penicillin allergy is common in clinical practice. Opting for non-beta-lactam antibiotics has its inconveniences and is often unnecessary, because most of these patients are in fact not allergic. This study aimed to determine how physicians in a large Canadian tertiary-care academic hospital without allergists on staff treat patients with a history of penicillin allergy. A retrospective study was conducted during a 1-year period among all patients hospitalized in the intensive care unit, coronary care unit, and internal medicine wards. Files of patients with a record of penicillin allergy were reviewed to assess the need for antibiotics during their hospitalization and the decision-making process underlying the choice of antibiotic. The additional costs of alternative antibiotics were calculated. The files of 1738 patients admitted over a 1-year period were hand reviewed. A history of penicillin allergy was found in 172 patients (9.9%). The allergic reaction was described in only 30% of cases and left unmentioned in 20.7%. Beta-lactam antibiotics were used on 56 occasions despite a history of penicillin allergy. The use of alternative antibiotics in place of the beta-lactam standard of care carried an additional cost of $15,672 Canadian. Alleged penicillin allergy is common among hospitalized patients and leads to substantial additional costs. Poor documentation of penicillin allergy likely reflects a lack of knowledge on this issue in the medical community, which impairs optimal treatment of these patients. Increased education on this matter is needed, and allergists on staff could be part of the solution. Copyright © 2013 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.