Rates of in vitro resistance to antituberculous drugs were examined for all patients hospitalized with active tuberculosis between Jan. 1979 and Dec. 1972, and between March 1975 and Sept. 1976. During the former period, in 31.5% of patients, tuberculosis was resistant to 1 or more drugs, and in 12.3% of patients was resistant to 2 or more drugs. During the latter period, 35.5% of patients had tuberculosis that was resistant to 1 or more drugs, and 19.6% had tuberculosis resistant to 2 or more drugs. Resistance to isoniazid and ethambutol increased significantly, whereas resistance to p-aminosalicytic acid decreased. Age, national origin and length of residence in the USA were not good predictors for the presence of in vitro resistance.