This study compared the short-term clinical effect of 0.12% chlorhexidine gluconate and placebo mouthrinses in 30 adolescents (ages 11 to 15) undergoing orthodontic treatment. Subjects were randomized into experimental (CHX) and control (C) groups. Baseline values were recorded 10 days after prophylaxis and included Plaque index (PI), Gingival index (GI), Rentention Index (RI), Discoloration index (DI), and probing depths (PD). Both groups (CHX and C) received soft toothbrushes with instructions to brush twice daily, as well as the CHX and placebo mouthrinses, respectively, with oral and written instructions for rinsing twice daily with 15 ml for 30 seconds. Reevaluations were performed 1, 2, and 3 months after baseline, except for the DI and PD, which were only assessed at 3 months. The Student's t test and the paired t test were used to analyze the data at the P < 0.05 level of significance. No differences between groups were seen at baseline for any of the parameters. At 30 days, there was a significant difference for the RI between CHX (0.15 +/- 0.16; mean +/- SD) and C (0.05 +/- 0.06) at the mesial buccal, and for CHX (0.07 +/- 0.10) and C (0.02 +/- 0.05) at the midbuccal. The 60-day evaluation showed similar results. At 90 days, lower PI were observed in the CHX group at the distal buccal (0.38 +/- 0.19), midbuccal (0.22 +/- 0.17), and mesial buccal (0.47 +/- 0.22) sites as compared with the C group (0.97 +/- 0.38, 0.83 +/- 0.40, and 0.95 +/- 0.43, respectively). A similar trend was noted with the GI, as the lower values were related to the CHX group. The changes of the PI and GI, at 30, 60, and 90 days, as analyzed by the paired t test, were statistically significant in the case of the experimental group, as the changes in the means were a reflection of significantly lower scores observed in the experimental group. After 3 months, the DI showed higher scores in the experimental group as compared with the control, but they were not statistically significant. Deeper PD were detected in the C group at 90 days, and they were statistically significant, except for the midlingual site. The RI did not show significant differences at 90 days, but higher values were recorded in the CHX group. The data indicate that the use of the CHX, in addition to regular oral hygiene habits, was effective in reducing plaque and gingivitis in adolescents undergoing orthodontic treatment.