The outcomes of tuberculosis (TB) patients who move before completing antituberculosis treatment have not been described. We studied a population-based cohort of 2,576 adult patients reported as having TB in California during 1993, including 147 patients who moved from one local health jurisdiction to another within California. We determined treatment outcomes (completed, defaulted, died, other) for 131 (89%) of these 147 patients. Patients who moved defaulted more often (relative risk [RR] = 5.5, 95% confidence interval [CI] = 4.1 to 7.4) than patients who did not move. Including these patients' treatment outcomes increased the known number of defaulters by 30%, from 141 to 183 persons. Additionally, diagnosis of TB in a state prison emerged as the strongest risk factor for defaulting from treatment. Patients who moved or defaulted were more likely to abuse drugs or alcohol, to be homeless or to be associated with congregate settings such as jails and prisons. On average, patients who defaulted after moving received less than three-quarters of their recommended treatment regimens. These patients may remain infectious or become infectious again. Our findings highlight the importance of ensuring complete treatment for TB patients who move; failure to do so will adversely affect patient health and TB control, especially in many high-risk populations and settings.