OBJECTIVES: To estimate the magnitude of antituberculous drug resistance and identify prospectively the risk factors for its development in tuberculosis (TB) patients in western Canada over a one-year period.DESIGN: Comparison of drug-resistant and nondrug-resistant cases of TB.SETTING: Western Canada.PATIENTS: All people with TB reported to the TB registries of Manitoba, British Columbia, Alberta and Saskatchewan between February 1, 1993 and January 31, 1994.MAIN OUTCOME MEASURES: Drug susceptibility testing was performed in all cases of culture-positive tuberculosis. Patients at risk for human immunodeficiency virus (HIV) infection were serotested.RESULTS: Of 534 culture positive cases of TB, 37 (6.9%) were drug resistant. Odds ratios suggested that the risk of drug resistance was significantly higher among those with reactivation than among those with new disease, and among those born outside of Canada than among those born in Canada. Ninety per cent of the foreign-born patients with drug-resistant disease were from Asia. Of the 35 patients with drug resistance whose type of resistance was known, 76% had initial and 24% had acquired drug resistance. The initial resistance rate in Asian-born patients was 14%. Most of the 37 drug-resistant cases were resistant to isoniazid (68%), streptomycin (49%) or both (22%). Twelve (32%) of the 37 cases were resistant to two or more first-line drugs. Of 14 patients who were HIV seropositive only one, a foreign-born patient, was drug resistant.CONCLUSION: Antituberculous drug resistance is low among Canadian-born patients in western Canada, but not uncommon among those born outside Canada. Initial therapy of foreign-born patients should include four first-line drugs.