Latent tuberculosis infection (LTBI) is a condition in which a person is infected with Mycobacterium tuberculosis, but does not currently have active tuberculosis disease. An estimated 10 to 15 million persons in the United States have LTBI. Because 5 to 10 percent of persons with LTBI are at risk of progressing to active disease, identification and treatment of LTBI are essential for the elimination of tuberculosis. Screening is recommended for high-risk persons, including immigrants; residents and employees of congregate living facilities; and persons infected with human immunodeficiency virus. Targeted tuberculin skin testing remains the most acceptable method of LTBI screening. New tests are being developed, the most promising of which are in vitro interferon-gamma release assays. All screened persons found to have LTBI should be offered treatment, regardless of age. Before initiating treatment, active tuberculosis must be ruled out by patient history, physical examination, and chest radiography. The treatment of choice for LTBI is isoniazid for nine months. Hepatotoxicity is the most severe adverse effect. Isoniazid should be discontinued if transaminase levels are greater than three times the upper limit of normal in symptomatic patients or five times the upper limit of normal in asymptomatic patients.