This study aimed to evaluate the effect of flossing on the presence of plaque and gingivitis in school students undertaking supervised oral hygiene. Thirty-three non-users of dental floss were selected from a larger panel. The study had a preliminary phase of 21 days, followed by two experimental phases of 21 days, separated by an interval of 21 days. The study was crossover and blind with two experimental groups. The addition of flossing to brushing was assigned to one group in each phase. Instructions in brushing and flossing were provided at the beginning, with re-instruction twice a week. The Visible Plaque Index (VPI) and the Gingival Bleeding Index (GBI) were determined at start and end of each phase. The medians were analyzed using the Wilcoxon and Mann-Whitney tests. In the first phase, the median interproximal GBI reduced from 62.1% to 17.9% with flossing, and from 59.3% to 22.2% without flossing. In the second phase, the median percentage of bleeding also reduced from 31.9% to 17.3% without flossing, and from 26.0% to 8.9% with flossing. The change in percentage was significant in both phases but similar for both groups. Similar results were observed in sites with VPI and sites where VPI and GBI were present. Free sites responded similarly to the interproximal sites. A regimen of brushing produced significant reductions in the presence of VPI and GBI both at free and interproximal sites; the inclusion of flossing into the regime did not represent significant improvements with respect to VPI and GBI. Keywords: dental floss, oral hygiene, brushing, gingivitis