This study was performed to evaluate the relation between the interval of supportive periodontal therapy and the prevalence of the subgingival microflora. The subgingival plaques from 108 patients were used in the study. Control group were the patients with no periodontal treatment and test groups were assigned into 3 groups according to the period of recall check: group 1; 1-2 months, group 2; 3-4 months, group 3; 6months or more. The polymerase chain reaction (PCR) used for direct identification of periodontal pathogens (P. gingivalis, T. forsythensis, T. denticola) in subgingival plaque. The results of this study were as follows. 1. The prevalence of P. gingivalis, T. forsythensis, T. denticola in control group were 100%, 87%, 90%. 2. In clinical parameters such as plaque index, gingival index, bleeding on probing, control group was not significant different with group 1 but Significant different with group 2, group 3. 3. In group 1, the majority of P. gingivalis had type II fimA. 4. When group 3 were compared with group 1, the prevalence of P. gingivalis increased. But the prevalence of P. gingivalis with type II fimA, which have the virulence factor, decreased. 5. We were unable to find the correlation between P. gingivalis with type IV fimA and periodontal disease. 6. The prevalence of T. forsythensis, T. denticola in test group were 85%, 93% or more. From the above results, we were able to find the relation between the interval of supportive periodontal therapy and the prevalence of the subgingival microflora and the need of the strict supportive periodontal therapy to prevent recurrence of periodontal disease, because there were high prevalence of periodontal pathogens.