Researchers did a study with 25 people. In the first part of the study, the group brushed their teeth first, then flossed. In the second part, they flossed first, then brushed. The researchers found that flossing first significantly reduced the amount of plaque between the teeth and from the mouth. Flossing loosened the plaque, then brushing and rinsing removed it. Bacteria in plaque are the main cause of periodontal disease. Periodontal disease starts when bacteria collects below the gum line. So the answer is: floss first, then brush and rinse. And see your dentist regularly.