Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Objective: This study assessed gene expression by quantitative polymerase chain reaction of inflammatory-[interleukin (IL)-12, tumor necrosis factor-alpha (TNF-alpha), IL-4, and IL-10] and osteoclastogenesis-related factors [receptor activator of NF-kappa B ligand (RANKL) and osteoprotegerin (OPG)] in sites exhibiting different severities of peri-implant disease. Material and methods: Peri-implant soft tissue biopsies (n=48) were harvested from healthy implant (HI), mucositis (MC), initial peri-implantitis (IP) and severe peri-implantitis (SP) sites. Results: IL-12 and TNF-alpha mRNA levels were higher in SP, followed by IP and MC (P < 0.05). IL-4 was higher in HI, followed by MC, SP and IP (P < 0.05). IL-10 was the lowest in HI, while no differences were detected among the diseased groups (P=0.05). OPG mRNA levels were higher in HI, followed by IP, SP and MC, whereas RANKL was increased as the peri-implantitis severity increased (P < 0.05). The highest OPG/RANKL ratio was observed in HI and the lowest in SP (P < 0.01). Conclusion: These findings suggest that expressions of inflammatory- and osteoclastogenesis-related factors may play an important role in the onset and severity of the peri-implant diseases.205514520Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [05/02561-3, 06/04604-4]CNPq [471282/2007-3, 303980/2007-9]