The worldwide emergence of extensively drug-resistant tuberculosis (XDR TB) and a provisional definition for this form of TB were first reported in November 2005. A more detailed description of these findings and preliminary data from the U.S. National TB Surveillance System (NTSS) were published in 2006. The U.S. data indicated that 74 TB cases reported during 1993-2004 met the case definition for XDR TB. Subsequent reports suggested different definitions for XDR TB. In October 2006, the World Health Organization convened an Emergency Global Task Force on XDR TB, which revised the case definition to specify resistance to at least isoniazid and rifampin among first-line anti-TB drugs, resistance to any fluoroquinolone, and resistance to at least one second-line injectable drug (amikacin, capreomycin, or kanamycin). This report updates the 2006 report on XDR TB in the United States, using the revised case definition and provisional data for 2006. NTSS data were analyzed for reported XDR-TB cases during 1993-2006; a total of 49 cases (3% of evaluable multidrug-resistant [MDR] TB cases) met the revised case definition for XDR TB. Of these, 17 (35%) were reported during 2000-2006. Compared with 1993-1999, cases from 2000-2006 were more likely to be in persons who were foreign born and less likely to be in persons with human immunodeficiency virus (HIV) infection. XDR TB presents a global threat and a challenge to TB-control activities in the United States. To prevent the spread of XDR TB, renewed vigilance is needed through drug-susceptibility testing, case reporting, specialized care, infection control, and expanded capacity for outbreak detection and response.