Objectives To estimate recent trends in CT/MRI utilisation among patients seeking emergency care for atraumatic headache in the USA and to identify factors associated with a diagnosis of significant intracranial pathology (ICP) in these patients.

Design/setting/participants Data were obtained from the USA National Hospital Ambulatory Medical Care Survey of emergency department (ED) visits between 1998 and 2008. A cohort of atraumatic headache-related visits were identified using preassigned ‘reason-for-visit’ codes. Sample visits were weighted to provide national estimates.

Results Between 1998 and 2008 the percentage of patients presenting to the ED with atraumatic headache who underwent imaging increased from 12.5% to 31.0% (p<0.01) while the prevalence of ICP among those visits decreased from 10.1% to 3.5% (p<0.05). The length of stay in the ED was 4.6 h (95% CI 4.4 to 4.8) for patients with headache who received imaging compared with 2.7 (95% CI 2.6 to 2.9) for those who did not. Of 18 factors evaluated in patients with headache, 10 were associated with a significantly increased odds of an ICP diagnosis: age ≥50 years, arrival by ambulance, triage immediacy <15 min, systolic blood pressure ≥160 mm Hg or diastolic blood pressure ≥100 mm Hg and disturbance in sensation, vision, speech or motor function including neurological weakness.

Conclusions The use of CT/MRI for evaluation of atraumatic headache increased dramatically in EDs in the USA between 1998 and 2008. The prevalence of ICP among patients who received CT/MRI declined concurrently, suggesting a role for clinical decision support to guide more judicious use of imaging.