The purpose of this study was to assess the prevalence of Tannerella forsythia in the primary infections of root canals and its association with different forms of periradicular diseases using Polymerase Chain Reaction (PCR).Samples were collected from acute and chronic forms of periradicular diseases and abscesses. DNA extraction was performed using spin column DNA isolation kit and 16S rRNA gene based PCR analysis was performed. Chi-squared test was used for data analysis.Tannerella forsythia DNA was detected in 18 (31,3%) of the total 58 samples, in 13 of 26 cases (50%) with acute apical periodontitis, three of seven cases (42,8%) with acute periradicular abscesses, and 2 of 25 cases (8%) with chronic asymptomatic periradicular lesions. It was found to be significantly related with acute apical periodontitis group (p=0.002).T. forsythia must be considered as a potential endodontic pathogen associated with acute symptoms of periradicular disease.