The most important cause of failure of antituberculosis therapy is that the patient does not take the medication as prescribed. To assess this problem, a retrospective review was conducted, using medical and nursing records, of adult patients treated at the tuberculosis clinic of the Montreal Chest Hospital in 1987-1988. In all, 352 patients were identified, of whom 59% were judged to have completed therapy. Completion of therapy was recorded in 92% of those with culture-positive disease, 76% of those with active but culture-negative disease and 54% among the 300 prescribed preventive therapy (P < 0.001). Compliance with preventive therapy was highest among those who had been in contact with an active case, and lowest among those identified through a workforce screening survey (P < 0.01). At the time of the first follow-up visit, patients identified to have suboptimal compliance were more likely to fail to complete therapy (P < 0.001). Compliance was higher among those initially hospitalized, those assessed to have better understanding (P < 0.05), those prescribed 6-9 rather than 12 months of therapy (P < 0.01), and those who returned for follow-up within 4 weeks of initiation of therapy (P < 0.01). Compliance could be improved by enhancing patient understanding, closer follow-up, and shorter therapy, particularly for those at lower risk of reactivation. As well, additional compliance enhancing interventions can be targeted to those patients with suboptimal compliance who can be accurately identified early in the course of therapy.