Pulmonary tuberculosis is a common infection worldwide. In the eastern part of Iran, fluctuations in tuberculosis prevalence are seen due to the migration of people from Afghanistan. The aim of this study was to evaluate the time of bacteriologic sputum conversion after treatment and affecting factors in those whose smear does not become negative.This study was carried out on 85 smear-positive pulmonary tuberculosis patients with the mean age of 65.6±16.7 years. There were 38 male patients (44.7%), 47 urban residents (55.3%), and only 6 patients (7.1%) from Afghanistan. The mean time of sputum conversion after starting treatment was 1.99±1.06 months. The mean time of sputum conversion was significantly higher in males (P=0.046), increased bacilli density in the primary sputum sample (P<0.0001, R=0.507), and pre-existing medical illness (P=0.001).In this study, it was revealed that pre-existing illness, bacilli density in initial smear, and severe involvement of the lung on radiography, could be associated with delay in sputum smear conversion.