Objective: The proportion of tuberculin reactors in a population and the intensity of tuberculin reactions have been shown to increase with increasing exposure to mycobacterial infection, eg, repeated BCG immunization. These observations suggested that tuberculin reactivity would become uniformly high in individuals with a high mycobacterial load who did not have a known cause of anergy. Since tuberculin reactivity has been measured to evaluate the possible genetic regulation of responses to mycobacteria in humans, it is important to study its behavior under conditions of ongoing, maximal exposure to mycobacteria. In the present study, we determined the mean size of tuberculin reactivity in BCG-immunized and unimmunized patients with pulmonary tuberculosis of recent onset, and the stability of tuberculin reactions during and after treatment of pulmonary tuberculosis., Method: Serial tuberculin testing was performed on patients with newly diagnosed active pulmonary tuberculosis diagnosed over a period of 2 years at the National Institute for Respiratory Diseases in Santiago, Chile. The first tuberculin test was performed at the time of diagnosis in 58 patients. Repeated tuberculin testing was performed 2 weeks later in 15 patients with initial reaction sizes < 15 mm. Four additional tuberculin tests were performed, one each at 3-months intervals in 42 patients regardless of the size of the initial tuberculin reaction., Results: Tuberculin reactions at entry had a unimodal distribution in patients both with and without BCG scars (14.8 +/- 5.0 mm and 16.5 +/- 5.2 mm, respectively). A second tuberculin test in patients with initial reaction sizes < 15 mm showed a moderate, statistically significant increase in the mean reaction size (PPD1: 10.1 +/- 3.2 mm; PPD2: 11.9 +/- 4.8 mm). Repeated tuberculin testing over 1 year revealed no significant changes in reaction size. The mean reaction sizes were 15.8 +/- 5.0 mm at entry, 15.5 +/- 5.4 mm at 3 months, 17.2 +/- 5.2 mm at 6 months, 17.0 +/- 5.1 mm at 9 months, and 16.7 +/- 54 mm at 12 months. The standard deviation of a random observation within patients was 5.3 mm. The largest variations due to increased reactivity after 6 months of treatment were observed in patients with reaction < 15 mm at entry compared with hyperergic patients, and in BCG-immunized patients compared to unimmunized patients., Conclusions: In the presence of an ongoing mycobacterial infection, patients without anergizing conditions express a tuberculin reactivity that is relatively constant during and after treatment of pulmonary tuberculosis. The size and stability of the reactions seem to be determined by individual conditions that include the tuberculin reactivity at the time of diagnosis and the BCG immunization status.