Introduction: Frequent users to emergency departments (EDs) are a diverse group of patients with a disproportionate number of ED presentations. This study aimed to compare sociodemographic and clinical characteristics of adult high system users (HSUs) and control groups in two provinces. Methods: Cohorts of HSUs were created for Alberta and Ontario by identifying the patients with the top 10% of ED presentations in the National Ambulatory Care Reporting System during April 2015 to March 2016. Random samples of patients not in the HSU groups were selected in each province as controls (4:1 ratio). Sociodemographic and presentation data (e.g., Canadian Triage and Acuity Scale [CTAS], disposition) were extracted and compared using separate logistic regression models. Results: In Alberta, 101,250 HSU patients made 686,918 ED presentations (median [med] = 5 interquartile range [IQR] 4,7 presentations per patient), compared with 401,923 controls who made 560,765 ED presentations (med = 1 IQR 1,2 per patient). HSUs were more likely to be female (odds ratio (OR) = 1.20 95% confidence interval (CI) 1.18,1.22), older (OR = 1.03 per 5y 95%CI 1.03,1.03), live closer to hospital (OR = 1.02 per 100km 95%CI 1.00,1.03), and be from the lowest income quintile (OR = 1.39 95%CI 1.37,1.42) than controls. In Ontario, 478,424 HSUs made 2,222,487 ED presentations (med = 4 IQR 3,5 per patient) and 1,714,037 controls made 2,114,070 ED presentations (med = 1 IQR 1,1 per patient). Ontario HSUs were also more likely to be female (OR = 1.13 95%CI 1.12,1.14), older (OR = 1.03 per 5y 95%CI 1.03,1.03), and from the lowest income quintile (OR = 1.41 95%CI 1.40,1.42) than controls, but were less likely to live closer to hospital (OR = 0.93 per 100km 95%CI 0.92, 0.93). Higher acuity was seen in Ontario (CTAS 1/2 vs. others OR = 1.05 95%CI 1.04,1.06) but not for Alberta (CTAS 1/2 vs others OR = 0.75, 95%CI 0.74,0.76). Discharges were less likely in the HSUs compared to controls (Alberta OR = 0.89 95%CI 0.88,0,90; Ontario OR = 0.65 95%CI 0.65,0.66). HSUs were more likely to leave without being seen (Alberta OR = 1.10 95%CI 1.07,1.13; Ontario OR = 1.37 95%CI 1.35,1.40) and against medical advice (Alberta OR = 1.47 95%CI 1.41,1.53; Ontario OR = 1.67 95%CI 1.63,1.71). Conclusion: HSUs were more likely to be female, older, and poorer than controls. Ontario HSUs had higher acuity than the other groups. Disposition differed for HSUs and controls. Further study is required to identify ways to safely reduce ED utilization by HSUs.