Bronchiolitis obliterans, a rare and life-threatening form of fixed obstructive lung disease, is known to be caused by exposure to noxious gases in occupational settings and has been described in workers in the microwave-popcorn industry who were exposed to artificial butter-flavoring chemicals, including diacetyl. In August 2004, the California Department of Health Services (CDHS) and Division of Occupational Safety and Health (Cal/OSHA) received the first report of a bronchiolitis obliterans diagnosis in a flavor-manufacturing worker in California. In April 2006, a second report was received of a case in a flavor-manufacturing worker from another company. Neither worker was employed in the microwave-popcorn industry; both were workers in the flavor-manufacturing industry, which produces artificial butter flavoring and other flavors such as cherry, almond, praline, jalapeno, and orange. Both workers had handled pure diacetyl, an ingredient in artificial butter and other flavorings, and additional chemicals involved in the manufacturing process. Studies have indicated that exposure to diacetyl causes severe respiratory epithelial injury in animals. Because the manufacture of flavorings involves more than 2,000 chemicals, workers in the general flavor-manufacturing industry are exposed to more chemicals than workers in the microwave-popcorn industry, which primarily uses butter flavorings. Food flavorings are designated "generally recognized as safe" when approved by the U.S. Food and Drug Administration; flavorings are not known to put consumers at risk for lung disease. This report describes the first two cases of bronchiolitis obliterans in flavor-manufacturing workers in California, the findings of the public health investigation, and the actions taken by state and federal agencies to prevent future cases of occupational bronchiolitis obliterans. To identify cases and reduce risk for lung disease from occupational exposure to flavorings, a timely, effective response is needed, including medical surveillance, exposure monitoring, and reduced exposure.