The purpose of this study was to determine whether conventional toothbrushing and twice daily use of a brush-on 0.4% stannous fluoride (SnF2) gel containing more than 90% available Sn2+ would be more effective for controlling plaque accumulation and gingivitis in the presence of orthodontic appliances than conventional toothbrushing alone. Consecutively treated adolescents who were to receive full-mouth fixed orthodontic appliances were assigned to one of two groups according to age and sex criteria. The first group (control, n = 35) used toothbrushing with a standard fluoride toothpaste, whereas the second group (treatment, n = 30) used toothbrushing supplemented with a 0.4% SnF2 gel used twice daily for the entire 18-month study period. Clinical assessments (Plaque Index, Gingival Index, bleeding tendency, and coronal staining) were made single blind before appliances were placed and 1, 3, 6, 9, 12, and 18 months after appliances were placed. Complete data were obtained for 32 control and 23 SnF2 gel subjects. The results indicated that the SnF2 gel group had significantly lower scores for plaque index (p < 0.01), gingival index (p < 0.001), and bleeding tendency (p < 0.001) at all examinations than did the control group. In the SnF2 group, one subject developed mild coronal staining, and two subjects developed moderate staining. We conclude that the use of a 0.4% SnF2 gel containing more than 90% available Sn2+ is an effective adjunct to mechanical tooth cleaning in preventing gingivitis in adolescents undergoing orthodontic treatment with fixed appliances.