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. Retrospective observational study of immigration detainees attending the Royal Darwin Hospital ED during the 2011 calendar year. Number of ED attendances and primary diagnoses. 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%). 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.