Objective To assess the early variation of clinical periodontal indexes and Porphyromonas gingivalis
（P.gingivalis） in subgingival plaques of orthodontic adults. Methods 11 orthodontic adults were selected. Clinical periodontal indexes of observed teeth were examined at three different time points: Before orthodontic treatment, the first month after treatment and the third month after treatment, and subgingival plaques were collected simultaneously at each time point. Clinical periodontal indexes included four ones: Plaque index（PLI）, sulcus bleeding index（SBI）, probing depth（PD）, and attachment loss. We used real -time quantitative polymerase chain reaction to detect and quantitate the number of total bacteria and P.gingivalis in each sample to obtain positive rate of P.gingivalis and the percentage of P.gingivalis in total bacteria. Results PLI and SBI of the first and third month were more than that of the baseline（P0.05） at each time point. Conclusion Fixed orthodontic appliances caused plaque accumulation, accordingly slight gingiva inflammation and the increasement of P.gingivalis occurred in the early stage, but none periodontitis was found.
