Many Mycobacterium tuberculosis infections are currently observed. Several of these infections are resistant to antifimic drugs such as Isoniazid (INH) and an insufficient dosing of the compound due to a lack of adherence to treatment can cause low levels of the drug especially in rapid acetylators. The number of rapid and slow acetylators in a population starting treatment for pulmonary tuberculosis was measured once they received the first dose of INH. A sample of urine was collected from these persons and the ratio acetylisoniazid/free Isoniazid (AcINH/INH) was measured. Sixty seven patients were rapid acetylators and 44 patients were slow acetylators. The group of rapid acetylators had an average AcINH/INH ratio value of 85 ± 9% and slow metabolizers had an average activity of 48 ± 13%. Both types of acetylators were present in important relative proportions in this population. It is thought that, when these differences are not taken into account, the acquired resistance to INH in mycobacterium can establish sooner in rapid acetylators and a more probable presence of toxicity can appear in slow metabolizers. In both cases, preventive measures should be provided.