Reviewing cases of medical error in the Harvard Medical Practice Study, the authors attempt to gain insight into factors associated with adverse events and the extent to which these events were preventable. The most common types of preventable events are technical errors, errors in diagnosis, failures of prevention, and medication errors. They found that two-thirds of adverse events were preventable. Diagnostic mishaps and late surgical failures had the highest rates of preventability. The authors found that 78% of fatal errors were preventable. The authors conclude by proposing areas to target to reduce iatrogenic injury.