Rush immunotherapy (RIT) accelerates the build-up phase of traditional IT. The biggest potential benefit of using RIT is decreased time to symptomatic improvement. However, aeroallergen RIT carries an increased risk of systemic reaction (SR) compared with traditional IT. This study was designed to assess the safety of a modified 1-day multiple aeroallergen RIT protocol. A retrospective chart review was performed of 138 patients from an outpatient, university-based allergy practice who underwent RIT between November 2007 and February 2011. The RIT protocol consisted of eight injections over 5 hours, and stopped at one 10-fold dilution below the maintenance vial. All patients were premedicated on the same day of RIT with prednisone and histamine 1 and 2 receptor blockers. Primary end point observed was rate of SR. One hundred thirty-eight patients received a total of 2911 RIT injections. Thirty- eight patients (28%) had SRs. The SR rate per injection was 1.3%. Most of the reactions (82%) occurred after the last dose of the protocol. No patients with SR had severe anaphylaxis requiring emergency department support or hospitalization. The post-RIT SR rate was within the range seen with traditional IT. Well-controlled asthmatic patients were not at increased risk of SR compared with nonasthmatic patients. Modification of RIT to end at one 10-fold dilution below the maintenance vial for multiple aeroallergen RIT did not significantly decrease the SR rate compared with other protocols that end at the maintenance vial. Unlike hymenoptera RIT, aeroallergen RIT continues to be associated with a high risk of SR compared with traditional IT.