Background:Odontogenic cysts are distinctive disorders that affect oral and maxillofacial tissues.The majority of odontogenic cysts such as dentigerous cysts and orthokeratinizedodontogenic cyst show an indolent course, whilst others demonstrate an aggressive behavior and may have higher recurrence rate like odontogenickeratocysts and glandular odontogenic cysts.Here wecompared FAK and PCNA expression in the previously mentioned cysts and attempted to correlate the findings to their variable clinical behaviors. Aim of the study:This study aimed to clarify the role of epithelial to mesenchymal transition in invasion potential, proliferation and aggressiveness of these lesions, which had been achieved. Methods: Double Immunostaining technique for FAK and PCNA was performed on the same tissue specimens using two types of chromogens. DAB chromogen was used for the assessment of nuclear expression of PCNA, while AEC chromogen was used for the assessment of cytoplasmic expression of FAK. Results: The greatest mean of the nuclear immunoexpression of PCNA was recorded in glandular odontogenic cyst group, followed by the odontogenickeratocyst, while the greatest mean area percent of the cytoplasmic expression of FAK was recorded in odontogenickeratocyst group, followed by the glandular odontogeniccyst. Conclusions:Odontogenickeratocyst has the highest epithelial to mesenchymal transition ability followed by glandular odontogenic cyst, while glandular odontogenic cyst has the highest proliferation activity followed by odontogenickeratocyst. On the other hand, orthokeratinizedodontogenic cyst and dentigerous cyst have the lowest epithelial to mesenchymal transition ability and proliferation activity, which can explain the diverse behavior of those odontogenic cysts.