Isoniazid is one of the most useful drugs for the treatment of tuberculosis. However, in the last few years treatment with this drug has become long, complicated, and occasionally inefficient because of the increasing number of cases with resistance. The aim of our work was to know the isoniazid resistance level of 1,496 Mycobacterium tuberculosis strains that were submitted to the Mycobacteria Reference Center, Córdoba, during the years 1993-2000. A total of 1,186 strains were from pulmonary sources and 196 from extrapulmonary sources; no source was provided for 114 strains. All M. tuberculosis strains were previously identified by different methods. For sensitivity testing, the BACTEC 460 TB system was used initially and then the ESP II system was also used. The control strains used in this study were ATCC27294 (sensitive to SM, RP, EB, and INH) and ATCC35822, resistant to INH. The overall resistance rate obtained was 14.8%, of which 2.3% and 1.4% accounted for primary and secondary resistance types, respectively. Ninety-six out of the 221 resistant strains were resistant to INH only, and 8 strains were resistant to INH plus SM. A total of 117 multiresistant strains were found. An apparent increase in the resistance rate to INH in the last few years was observed. Secondary resistance has decreased, whereas primary resistance has increased. A continuous surveillance of resistance is required and, therefore, any patient with tuberculosis should undergo sensitivity testing to confirm therapy to be used.