Study objective Allergic reactions are common presentations to the emergency department (ED). An unknown proportion of patients will develop biphasic reactions, and patients are often monitored for prolonged periods to manage potential reactions. We seek to determine the incidence of clinically important biphasic reactions. Methods Consecutive adult patients presenting to 2 urban EDs with allergic reactions during a 5-year period were identified. Encounters were dichotomized as “anaphylaxis” or “allergic reaction” with an explicit algorithm. A comprehensive chart review was conducted on each index and all subsequent visits to detail patient presentations, comorbidities, ED management, and predefined clinically important biphasic reactions. Regional and provincial databases were linked to identify subsequent ED visits and deaths within a 7-day period. The primary outcome was the proportion of patients with a clinically important biphasic reaction, and the secondary outcome was mortality. Results Of 428,634 ED visits, 2,819 (0.66%) encounters were reviewed (496 anaphylactic and 2,323 allergic reactions). Overall, 185 patients had at least 1 subsequent visit for allergic symptoms. Five clinically important biphasic reactions were identified (0.18%; 95% confidence interval CI 0.07% to 0.44%), with 2 occurring during the ED visit and 3 postdischarge. There were no fatalities (95% CI 0% to 0.17%). In the anaphylaxis and allergic reaction groups, clinically important biphasic reactions occurred in 2 patients (0.40%; 95% CI 0.07% to 1.6%) and 3 patients (0.13%; 95% CI 0.03% to 0.41%), respectively. Conclusion Among ED patients with allergic reactions or anaphylaxis, clinically important biphasic reactions and fatalities are rare. Our data suggest that prolonged routine monitoring of patients whose symptoms have resolved is likely unnecessary for patient safety.