Liquid manure storage is a common component of confinement systems for swine, beef, dairy, and veal operations. A mail questionnaire to a sample of swine producers indicated that more than 85,000 people in Iowa and an estimated 500,000 in the United States work in livestock confinement systems that use liquid manure storage. Deaths and illnesses in people with acute exposure to toxic gases emanating from the liquid manure have been recently reported. This communication reports results of the investigation of six such incidents. Hydrogen sulfide appears to be the main toxic substance involved, and agitation of the liquid manure is important in creating an acutely severely toxic environment. Preventive measures must include worker education and limitation of human exposure through control of environmental and human factors.