Completion of treatment is key to tuberculosis control. Using national surveillance data we assessed factors associated with tuberculosis patients being lost to follow-up before completing treatment ('lost'). Patients reported in England, Wales and Northern Ireland between 2001 and 2007 who were lost 12 months after beginning treatment were compared to those who completed, or were still on treatment, using univariable and multivariable logistic regression. Of 41 120 patients, men [adjusted odds ratio (aOR) 1·29; 95% confidence interval (CI) 1·23-1·35], 15- to 44-year-olds (P<0·001), and patients with pulmonary sputum smear-positive disease (aOR 1·25, 95% CI 1·12-1·45) were at higher risk of being lost. Those recently arrived in the UK were also at increased risk, particularly those of the White ethnic group (aOR 6·39, 95% CI 4·46-9·14). Finally, lost patients had a higher risk of drug resistance (aOR 1·41, 95% CI 1·17-1·69). Patients at risk of being lost require enhanced case management and novel case retention methods are needed to prevent this group contributing towards onward transmission.