Abstract: Objective To evaluate the subgingival prevalence of human cytomegalorirus (HCMV),Epstein-Barr virus-1 (EBV-I) in chronic periodontitis (CP) patients before and after treatment and to analyze the relationship between the prevalent variance and periodontal clinical parameters.Methods Gingival crericular fluids of 13 CP patients were collected at baseline,2 weeks,2 months and 4 months after periodontal mechanical treatment.HCMV and EBV-1 were detected using nested polymerase chain reaction (n-PCR).Results The plaque index (PLI),probing depth (PD) and bleeding index (BI) of CP patients at 2 months,4 months after periodontal mechanical treatment were eridenfly lower than before treatment,P<0.01.These parameters at 4 months after treatment were higher than at 2 months,the differences were significant,P < 0.05.The prevalence of HCMV and EBV in CP patients was 42% (33/78),14% (11/78).EBV and HCMV were mostly coexistent in the same site[9 sites HCMV (+) in 11 EBV positive sites].The sites of HCMV(+) and EBV(+) were almost deep pockets.Thirteen of 14 sites with deep pocketswere HCMV(+),9 sites were deep pockets in 11 sites EBV(+).The prevalence of HCMV and EBV(8% and O respectively) at 2 weeks was the lowest in all four time points.The prevalence of HCMV and EBV at 2 weeks,2 months and 4 months following treatment was significantly lower than baseline(P < 0.01),but the prevalence of HCMV (15%) at 2 months after treatment was higher than at 2 weeks (8%),thedifference was not significant(P =0.133).Conclusions Herpesviruses may play a role in the developmentof CP.The changes of the prevalence of herpesviruses before the changes of clinical parameters could be detected after periodontal mechanical treatment.The patients should be re-evaluated and re-treated within 2 months after treatment.