OBJECTIVE: To describe the epidemiology of nontraumatic headache in adults presenting to emergency departments (EDs). BACKGROUND: Headache is a common reason for presentation to EDs. Little is known about the epidemiology, investigation, and treatment of nontraumatic headache in patients attending EDs internationally. METHODS: An international, multicenter, observational, cross-sectional study was conducted over one calendar month in 2019. Participants were adults (≥18 years) with nontraumatic headache as the main presenting complaint. Exclusion criteria were recent head trauma, missing records, interhospital transfers, re-presentation with same headache as a recent visit, and headache as an associated symptom. Data collected included demographics, clinical assessment, investigation, treatment, and outcome. RESULTS: We enrolled 4536 patients (67 hospitals, 10 countries). "Thunderclap" onset was noted in 14.2% of cases (644/4536). Headache was rated as severe in 27.2% (1235/4536). New neurological examination findings were uncommon (3.2%; 147/4536). Head computed tomography (CT) was performed in 36.6% of patients (1661/4536), of which 9.9% showed clinically important pathology (165/1661). There was substantial variation in CT scan utilization between countries (15.9%-75.0%). More than 30 different diagnoses were made. Presumed nonmigraine benign headache accounted for 45.4% of cases (2058/4536) with another 24.3% classified as migraine (1101/4536). A small subgroup of patients have a serious secondary cause for their headache (7.1%; 323/4536) with subarachnoid hemorrhage (SAH), stroke, neoplasm, non-SAH intracranial hemorrhage/hematoma, and meningitis accounting for about 1% each. Most patients were treated with simple analgesics (paracetamol, aspirin, or nonsteroidal anti-inflammatory agents). Most patients were discharged home (83.8%; 3792/4526). In-hospital mortality was 0.3% (11/4526). CONCLUSION: Diagnosis and management of headache in the ED is challenging. A small group of patients have a serious secondary cause for their symptoms. There is wide variation in the use of neuroimaging and treatments. Further work is needed to understand the variation in practice and to better inform international guidelines regarding emergent neuroimaging and treatment.