Concerns have been raised about whether workers exposed to engineered nanoparticles are at increased risk of adverse health effects. The current body of evidence about the possible health risks of occupational exposure to engineered nanoparticles is quite small. While there is increasing evidence to indicate that exposure to some engineered nanoparticles can cause adverse health effects in laboratory animals, no health studies of workers exposed to the few engineered nanoparticles tested in animals have been published. The purpose of this document from the National Institute for Occupational Safety and Health (NIOSH) is to provide interim guidance about whether specific medical screening, including performing medical tests on asymptomatic workers, is appropriate for these workers. Medical screening is only one part of what should be considered a complete safety and health management program. An ideal safety and health management program follows a hierarchy of controls and involves various occupational health surveillance measures. Since specific medical screening of asymptomatic workers exposed to engineered nanoparticles has not been extensively discussed in the scientific literature, this document makes recommendations based upon what is known until more rigorous research can be performed. Currently there is insufficient scientific and medical evidence to recommend the specific medical screening of workers potentially exposed to engineered nanoparticles. Nonetheless, this lack of evidence does not preclude specific medical screening by employers interested in taking precautions beyond existing industrial hygiene measures. If nanoparticles are composed of a chemical or bulk material for which medical screening recommendations exist, these same screening recommendations would be applicable for workers exposed to engineered nanoparticles as well. As research into the hazards of engineered nanoparticles continues, vigilant reassessment of available data is critical to determine whether specific medical screening is warranted for workers. In the interim, the following recommendations are provided for workplaces where workers may be exposed to engineered nanoparticles in the course of their work: Take prudent measure to control exposures to engineered nanoparticles; Conduct hazard surveillance as the basis for implementing controls; Continue use of established medical surveillance approaches. Concerned individuals from government, industry, labor, and academia, together with occupational health professionals and medical personnel, have raised questions about whether workers exposed to engineered nanoparticles should be provided some type of medical surveillance. The purpose of this document is to provide interim guidance concerning specific medical screening for these workers-that is, medical tests for asymptomatic workers-until additional research either supports or negates the need for this type of screening. The type and degree of screening recommended here is in addition to any medical surveillance taking place as part of existing occupational health surveillance efforts.