The role of the physician epidemiologist, in general, and those employed by the National Institute for Occupational Safety and Health (NIOSH) in particular, is described. Over a period of 4 years, the number of NIOSH physicians with epidemiological training has increased from 11 to 29, which represents 64.4 percent of all the physicians working for the agency. The role of the physician epidemiologist in the NIOSH Health Hazard Evaluation Program is described in detail and examples of such evaluations are reviewed. Generally, a Health Hazard Evaluation is triggered when a large proportion of the workers employed at a given facility demonstrate apparent excess morbidity, disability, or mortality of unknown cause; or the workers suspect that a given agent at work may be hazardous, without actually being aware of any adverse effects. Based on the outcome of the evaluation, recommendations are made for changes in exposure limits, safety practices, etc. The nature of the evaluations is illustrated by two examples, one involving the assessment of hazards at a storage battery plant, and the other, the evaluation of the eventual health effects resulting from the operation of a manual duplicating machine. It is pointed out that the NIOSH investigators are not restricted to the study of workers with known exposure to hazardous occupational agents or processes and are actually involved in the evaluation of the entire workforce of a given organization, whether exposed or nonexposed.