Objective: To assess emergency department (ED) doctors' knowledge of radiation doses associated with diagnostic imaging and to describe their practice with regard to informing patients of risk. Design, participants and setting: Prospective, questionnaire-based observational study in May 2009 among all 110 doctors in the EDs of a 570-bed teaching hospital and a 200-bed district hospital. Main outcome measures: Percentage knowledge score; and frequency of discussing radiation risk with patients, based on responses to three scenarios rated on a visual analogue scale (VAS), where a score of 100 indicates doctors would always discuss it. Results: 96 doctors (87%) completed the questionnaire. The overall mean knowledge score was 40% (95% CI, 38%-43%). Senior doctors scored somewhat higher than junior doctors, but not significantly (42% v 39%; P = 0.75). Over three-quarters of doctors (78%) underestimated the lifetime risk of fatal cancer attributable to a single computed tomography scan of the abdomen. Most doctors (76%) reported never having had any formal training on risks to patients from radiation exposure. The frequency at which doctors would inform patients of the risk of radiation varied greatly depending on the clinical scenario (mean VAS scores, between 38 and 90). Conclusion: Emergency doctors in our sample had a varied knowledge of the risks from radiation exposure, but overall knowledge was poor. Staff should receive education, and the diagnostic imaging request process may need to include information on radiation doses and risks.