Objective: To describe the number and nature of emergency department (ED) attendances by immigration detainees in Darwin, in the Northern Territory, over a 12-month period. Design and setting: Retrospective observational study of immigration detainees attending the Royal Darwin Hospital ED during the 2011 calendar year. Main outcome measures: Number of ED attendances and primary diagnoses. Results: In 2011, there were 770 ED attendances by 518 individual detainees at Royal Darwin Hospital. Those who attended the ED had a mean (SD) age of 27.6 (12.2) years, and 112 of them (21.6%) were children. Most (413, 79.7%) were male, and Iran and Afghanistan were the two most common countries of birth. We estimate that 50.1% (95% CI, 47.0%-53.2%) of immigration detainees in Darwin (mean, 776 per month; total, 1034), attended the Royal Darwin Hospital ED at least once in 2011. The most common primary diagnosis was psychiatric problems (187 attendances, 24.3%), including self-harm (138 attendances, 17.9%). Conclusion: In 2011, asylum seekers in immigration detention in Darwin had a high prevalence of unmet health needs and substantial levels of psychiatric morbidity. The primary health care provided to them was inadequate.