BACKGROUND:Currently, there are a lot of studies addressing oral microflora variation in patients wearing fixed orthodontic appliance. OBJECTIVE:To determine the changes in Streptococcus mutans, Lactobacil us and oral health in teenagers wearing metal orthodontic appliances. METHODS:Twenty-three who were subjected to fixed metal orthodontic appliances as testing group and 18 adolescent volunteers with no oral diseases served as controls. We col ected saliva samples from al the participants immediately, 90 days and 180 days after fixed metal orthodontic treatment to detect changes in Streptococcus mutans and Lactobacil us, which can lead to dental caries, and other oral parameters, including gingival index and oral hygiene index. RESULTS AND CONCLUSION:In the control group, there were no difference in Streptococcus mutans, Lactobacil us, gingival index and oral hygiene index at three time points. In the testing group, the hazard levels of Streptococcus mutans and Lactobacil us were significantly increased at 90 and 180 days than immediately after fixed metal orthodontic treatment (P<0.05), as wel as the gingival index at 90 days and oral hygiene index at 180 days became worse (P<0.05). At 180 days after fixed metal orthodontic treatment, the hazard levels of Streptococcus mutans and Lactobacil us, gingival index and oral hygiene index were severer in the testing group than the control group (P<0.05). These findings indicate that the fixed metal orthodontic appliance could lead to increasing of Streptococcus mutans and Lactobacil us, and reduce oral health status badly.