en Epidemiology Epidemiology Original Research Original Research <strong>Background: </strong>The mouth cavity hosts various types of anaerobic bacteria including Porphyromonas gingivalis, which causes periodontal inflammatory diseases. P. gingivalis is a gram-negative oral anaerobe and is considered as a main etiological factor in periodontal diseases. Several studies have reported a relationship between P. gingivalis in individuals with periodontal diseases and a critical role of this bacterium in the pathogenesis of periodontal diseases. The present study aimed at estimating this probability using a meta-analysis.<br> <strong>&nbsp;&nbsp; Methods:</strong> We searched several databases including PubMed, Scopus, Google Scholar, and Web of Science to identify case-control studies addressing the relationship between P. gingivalis with periodontal diseases. A total of 49 reports published from different countries from 1993 to 2014 were included in this study. I&sup2; (heterogeneity index) statistics were calculated to examine heterogeneity. Data were analyzed using STATA Version 11.<br> <strong>&nbsp;&nbsp; Results:</strong> After a detailed analysis of the selected articles, 49 case-control studies with 5924 individuals fulfilled the inclusion criteria for the meta-analysis. The healthy controls included 2600 healthy individuals with a Mean&plusmn;SD age of 36.56&plusmn;7.45 years. The periodontal diseases group included 3356 patients with a mean age of 43.62&plusmn;8.35 years. There was a statistically significant difference between P. gingivalis in periodontal patients and healthy controls; 9.24 (95% CI: 5.78 to 14.77; P = 0.000). In the other word, there was a significant relationship between the presence of P. gingivalis and periodontal diseases.<br> <strong>&nbsp;&nbsp; Conclusion:</strong> Analyzing the results of the present study, we found a strong association between the presence of P. gingivalis and periodontal diseases. This result suggests that another research is needed to further assess this subject.<br> &nbsp; Porphyromonas gingivalis, Periodontal Diseases, Chronic Periodontitis, Aggressive Periodontitis, Gingivitis 355 362 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-1-1115&slc_lang=en&sid=1 Mohammad Rafiei 200319475328460035826 200319475328460035826 No Department of Biostatistics and Epidemiology, School of Medicine, Arak University of Medical Sciences, Arak, Iran. Faezeh Kiani 200319475328460035827 200319475328460035827 No Student Research Committee, Ilam University of Medical Sciences, Ilam, Iran. Fatemeh Sayehmiri 200319475328460035828 200319475328460035828 No Proteomics Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Kourosh Sayehmiri 200319475328460035829 200319475328460035829 No Department of Social Medicine, School of Medicine, Ilam University of Medical Sciences, Ilam, Iran. Abdolkarim Sheikhi sheikhi.a@dums.ac.ir 200319475328460035830 200319475328460035830 Yes Department of Immunology and Microbiology, Dezful University of Medical Sciences, Dezful, Iran. Mona Zamanian Azodi 200319475328460035831 200319475328460035831 No Proteomics Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran.