Highly frequent users (HFU) of the emergency department (ED) are a poorly defined population. This study describes patient and visit characteristics for Canadian ED HFU and patient subgroups with mental illness, substance misuse, or ≥ 30 yearly ED visits. We reviewed health records from a random selection of adult patients whose visit frequency comprised the 99th percentile of yearly ED visits to The Ottawa Hospital. We excluded scheduled repeat ED assessments. We collected the following: 1) patient characteristics - age, sex, and comorbidities; and 2) ED visit characteristics - diagnosis category, length of stay, presentation time, consultation services, and final disposition. Two reviewers collected data, and we performed an inter-rater review to measure agreement. We analyzed 3,164 ED visits for 261 patients in all subgroups overall. Within the HFU random selection, mean age was 53.4 ± 1.3, and 55.6% were female. Most patients had a fixed address (88.9%), and family physician (87.2%). Top ED diagnoses included musculoskeletal pain (9.6%), alcohol intoxication (8.5%), and abdominal pain (8.4%). Allied health (social work, geriatric emergency medicine, or community care access centre) was consulted for 5.9% of visits. In 52.7% of these cases, allied health services were not available at the time of presentation. HFU are a complex population who represent a marked proportion of annual ED visits. Our data indicate that there are opportunities to improve the current approaches to care. Future work examining ED-based screening and multi-disciplinary approaches for HFU may help reduce frequent ED presentations, and better serve this vulnerable population.