BACKGROUND Extensively drug-resistant (XDR) tuberculosis (TB) is a global public health emergency. We investigated the characteristics and extent of XDR TB in California to inform public health interventions. METHODS XDR TB was defined as TB with resistance to at least isoniazid, rifampin, a fluoroquinolone, and 1 of 3 injectable second-line drugs (amikacin, kanamycin, or capreomycin). Pre-XDR TB was defined as TB with resistance to isoniazid and rifampin and either a fluoroquinolone or second-line injectable agent but not both. We analyzed TB case reports submitted to the state TB registry for the period 1993-2006. Local health departments and the state TB laboratory were queried to ensure complete drug susceptibility reporting. RESULTS Among 424 multidrug-resistant (MDR) TB cases with complete drug susceptibility reporting, 18 (4.2%) were extensively drug resistant, and 77 (18%) were pre-extensively drug resistant. The proportion of pre-XDR TB cases increased over time, from 7% in 1993 to 32% in 2005 (P = .02)). Among XDR TB cases, 83% of cases involved foreign-born patients, and 43% were diagnosed in patients within 6 months after arrival in the United States. Mexico was the most common country of origin. Five cases (29%) of XDR TB were acquired during therapy in California. All patients with XDR TB had pulmonary disease, and most had prolonged infectious periods; the median time for conversion of sputum culture results was 195 days. Among 17 patients with known outcomes, 7 (41.2%) completed therapy, 5 (29.4%) moved, and 5 (29.4%) died. One patient continues to receive treatment. CONCLUSIONS XDR TB and pre-XDR TB cases comprise a substantial fraction of MDR TB cases in California, indicating the need for interventions that improve surveillance, directly observed therapy, and rapid drug susceptibility testing and reporting.