Introduction: The immediate identification of infectious tuberculosis and implementation of effective isolation measures, are a priority of the national policies for disease control. Objectives: To identify clinical predictor's factors related with sputum smear-positive, in patients with pulmonary tuberculosis diagnosis. Population and methods: This was a retrospective study involving 289 patients admitted consecutively with the diagnosis of tuberculosis (ICD-9-CM: 010-018) through the emergency department of a Central Hospital. The study was conducted from January 1999 to December 2005. Results: 216 patients (74.7%) were identified with pulmonary tuberculosis: mean age 40.3 (SD 16.1); 77.8% males. Of these 179 (82%) were smear-positive and 157 (54.5%) had human immunodeficiency virus (HIV) co-infection. Of the 37 patients with sputum smearnegative 18 (48.6%) had culture positive. The clinical factors that were better related with sputum smear-positive were: cough with sputum (p = 0.031) and classic chest radiograph patterns, parenchymal consolidation (p < 0.001) and cavitation (p = 0.002). HIV co-infection wasn't associated with a higher risk of infectious tuberculosis. Conclusions: Respiratory symptoms and classic patterns on chest radiograph, were clinical factors better related with a higher probability of sputum smear-positive. In these patients it is a good clinical practice to propose immediate isolation measures even before the results of sputum smear.