The purpose of this retrospective study was to assess cross-hypersensitivity between imipenem/cilastatin and penicillin in patients with reported penicillin allergies. Medical records of febrile neutropenic, penicillin-allergic bone marrow transplant recipients who received imipenem/cilastatin treatment were retrospectively reviewed. The findings of this study indicate the incidence of cross-reactivity between imipenem/cilastatin and penicillin among patients with a history of penicillin allergy may be lower than previously reported.