Pegaspargase (PEG) is a standard component of therapy for pediatric acute lymphoblastic leukemia (ALL). Because PEG preparations are bacterially derived, they are highly immunogenic. PEG has traditionally been delivered intramuscularly (IM), but over the last several years, more PEG has been given intravenously (IV) in order to provide a less painful and more convenient means of delivery. However, there are limited data comparing allergic reactions between IV and IM PEG recipients, especially in a large cohort of patients. We reviewed the charts of pediatric ALL patients diagnosed from 2006 to 2011 who received PEG at our institution and compared the incidence, time to onset of symptoms, reaction grade, and hospitalization rate for patients who had allergic reactions to PEG. Of 318 evaluable patients, 159 received IV and 159 received IM PEG. Thirty-one (19.5%) IV patients had an allergic reaction, compared to 17 (10.7%) IM patients (P = .028). Time to onset of symptoms was ≤ 30 minutes for 26 of 27 evaluable IV patients (96.3%) versus only two of 11 evaluable IM patients (18.2%; P < .001). Four of 31 IV patients (12.9%) and six of 17 IM patients (35.5%) required hospitalization (P = .134). There is increased incidence of allergy in patients who received IV PEG compared to IM. Grade of reaction was similar between IV and IM, but allergic reactions to IV PEG had a more rapid onset. While the risk of allergy may be increased, IV delivery appears to have an acceptable safety profile for administration in ALL patients.