From july 1982 to december 1987 we prospectively evaluated the primary resistance (PR) of 248 strains of M. tuberculosis isolated from patients with several clinical forms of tuberculosis. The results were compared with those of a similar study carried out between 1978-1982. PR persisted relatively stable with time, although it showed a trend towards reduction. Isoniazide resistance (4.8%) still has a leading epidemiological interest. The 89% of strains showed isolated resistance to a single drug. Only two strains had associated resistance to INH and SM. Strains with PR to RIF, ETB or PZA were not detected. Of the 248 evaluated strains, 32 were isolated from intravenous drug abusers with AIDS. Only one of these strains had a low level (0.2 mg/l) PR to INH. At last two of these drug abusers developed acquired resistance to INH and ETB-RIF, respectively. Considering the difficulty to control chemotherapy in this group of patients, our finding has a considerable epidemiological interest.