Objective: To describe the incidence, type, causes and consequences of nuclear medicine maladministrations. Setting and participants: Review of prospectively acquired maladministration reports within the Australian Radiation Incident Register (ARIR), a mandatory incident register managed by the Australian Radiation Protection and Nuclear Safety Agency. Main outcome measures: Individual reports from 2007 to 2011 were evaluated for dose of radiation exposure and type, cause and consequence of maladministrations. Incidence was estimated using data from Medicare Australia. Results: There were 149 maladministrations and the estimated incidence was 5.8 per 100 000 nuclear medicine procedures (95% CI, 5.0-6.9). About half of all maladministrations (48%) arose from an incorrect radiopharmaceutical being prepared and/or dispensed. Other causes included mistakenly injecting the wrong radiopharmaceutical because of inattention (n=27; 18.1%); extravasations, failures in equipment or procedure leading to a non-diagnostic study (n=25; 16.8%); misinterpreting a request form and performing an incorrect procedure (n=13; 8.7%); or injecting an incorrect patient (n=13; 8.7%). ARIR reports focused on active rather than latent causes. Most (n=147) maladministrations occurred following diagnostic procedures, and the mean effective radiation dose was 7.9mSv (range, 0.015-45mSv). Two therapeutic maladministrations likely caused unintended organ injury. Conclusions: The ARIR provides unique insight into the type, causes and complications of maladministrations in Australia. Nearly all maladministrations occur in a diagnostic context, and the risk of patient harm appears low. Among active causes, radiopharmaceutical preparation and dispensation, and medical supervision before injection merit attention. The ARIR could be refined by attending to latent errors, addressing possible underreporting and securing more complete Medicare data.