Hospital-based occupational tuberculosis (TB) control programs have four basic components: rapid detection of TB disease in presenting patients; use of environmental controls, including personal respiratory protection; periodic tuberculin skin testing; and administration of prophylactic antibiotic therapy to newly infected employees. This article assesses which component is the most important in reducing TB disease risk among health care workers. A quantitative framework for estimating disease risk is developed, and two important results are described. First, the rapid identification of TB disease in presenting patients is the most important element in the overall program. Second, once TB disease has been identified, the use of highly efficient environmental controls (which include respiratory protection) becomes the most important element; these controls are especially important for procedures such as bronchoscopy and autopsy, which can aerosolize large numbers of viable Mycobacterium tuberculosis bacilli.