Introduction: The Calgary Stampede is a two-week mass gathering occurring annually in July. Clinicians have anecdotally noted increases in emergency department (ED) and urgent care (UC) visits, especially for complaints related to substance misuse and violence. Our objectives were: 1) to determine if there is an increase in overall visits to EDs and UCs during the Stampede, and 2) to determine if there are increases in presentations related to trauma, violence, or intoxication. Methods: This observational study used prospectively collected administrative data from five EDs and two UCs in Calgary. For the years 2013 to 2016, daily average data during Stampede dates were compared to the data from the 21 days immediately preceding and following the event. Dates were selected to incorporate a similar proportion of weekends and weekdays in the Stampede and non-Stampede periods. The primary outcome was daily average ED and UC utilization. Secondary outcomes included time of arrival, utilization by demographic groups, complaint category at triage, or International Statistical Classification of Diseases, 10th revision (ICD-10) diagnosis. Results: The study period included 263 380 individual ED and UC visits (34 492 Stampede and 228 888 non-Stampede visits). Daily average ED and UC visits increased by 2.1% (p<0.0001) during the Stampede period. Increases in utilization were identified in specific subgroups: male, ambulance arrival, and nighttime arrival between 2000 and 0400 (all p<0.05). The Stampede period saw a marked increase in CTAS 1 visits (16.2%, p<0.01), triage complaints of lacerations (12.4%, p<0.0001) and blunt trauma (19.4%, p<0.0001), and the ICD-10 diagnosis of substance misuse (23.9%, p=0.01). Visits triaged to the minor treatment areas increased by 9.5% (p<0.0001), again most markedly at night (15.3%, p<0.0001).No differences were detected for triage complaints of altered level of consciousness, sexual assault, head or neck injury, limb injury, or social problems. Conclusion: The Calgary Stampede provokes appreciable changes in overall ED and UC utilization, with marked increases in nighttime visits, visits by men, trauma or substance abuse-related complaints, and minor treatment visits. This data may be useful in manpower planning to ensure optimal patient flow and service delivery during mass gatherings.