Unplanned releases of ammonia lead more often to evacuation and injury than releases of other chemicals, but few studies have systematically investigated ammonia releases. We analyzed Hazardous Substances Emergency Events Surveillance system data for 1993-1998. Evacuation of a total of at least 40,680 persons resulted from 537 ammonia releases, and 248 ammonia releases led to injury of 1434 persons. Equipment failure and operator error were cited as factors contributing to ammonia releases 90% of the time. Eighty-seven percent of releases occurred at fixed facilities. Risk factors for evacuation and injury differed between the food-manufacturing industry and other industries. Indoor release was a consistent risk factor, whereas quantity of ammonia released was not always a risk factor. Preventive maintenance and worker training may be effective tools to reduce the burdens of hazardous ammonia releases.