Introduction: Emergency department (ED) boarding is associated with worse outcomes for critically ill patients. There have been mixed findings in other patient populations. The primary objective of this study was to examine predictors of prolonged ED boarding among cancer patients receiving chemotherapy who required hospital admission from the ED. Secondary objectives were to examine the association between prolonged ED boarding and in-hospital mortality, 30-day mortality, and hospital length of stay (LOS). Methods: Using administrative databases from Ontario, we identified adult (≥ 18 years) cancer patients who received chemotherapy within 30 days prior to a hospital admission from the ED between 2013 to 2017. ED boarding time was calculated as the time from the decision to admit the patient to when the patient physically left the ED. Prolonged ED boarding was defined as ≥ 8 hours. Multivariable logistic regression was used to examine predictors of prolonged ED boarding and to determine if prolonged boarding was associated with mortality. Multivariable quantile regression was used to determine the association between prolonged boarding and hospital LOS. Results: 45,879 patients were included in the study. Median (interquartile range (IQR)) ED LOS of stay was 11.8 (7.0, 21.7) hours and median (IQR) ED boarding time was 4.2 (1.6, 14.2) hours. 17,053 (37.2%) patients had prolonged ED boarding. Severe ED crowding was the strongest predictor of prolonged ED boarding (odds ratio: 17.7, 95% CI: 15.0 to 20.9). Prolonged ED boarding was not associated with in-hospital mortality or 30-day mortality. Median hospital LOS was over 9 hours (p <0.0001) longer among patients with the longest ED boarding times. Conclusion: Severe ED crowding was associated with a significant increase in the odds of prolonged ED boarding. While our study demonstrated that prolonged boarding was not associated with increased mortality, further work is required to understand if ED boarding is associated with other adverse outcomes in this immunocompromised population.