We determined the quantitative levels and the genetic mechanisms of resistance in drug-resistant clinical isolates of Mycobacterium tuberculosis sampled over a period of 3 years (n = 45; 17 of the isolate were multidrug resistant). Our results led us to hypothesize that some strains categorized as resistant to isoniazid, ethambutol, or streptomycin by standard laboratory procedures of in vitro drug susceptibility testing may still respond to a treatment regimen that includes these agents.