Background. One strategy for treating dental caries is to suppress oral mutans streptococci, or MS, with chlorhexidine, or CHX, mouthrinse. Oral MS levels, however, tend to quickly return to baseline values without further intervention. In this clinical study, the authors evaluated the effect of xylitol chewing gum on MS regrowth. Methods. The authors selected 151 subjects with elevated oral MS levels (≥ 105 colony-forming units per milliliter, or CFU/mL, of paraffin-stimulated saliva). Subjects rinsed with 0.12 percent CHX gluconate mouthrinse twice daily for 14 days. The authors then randomly assigned the subjects to one of three groups. Those in the test group (n = 51) chewed a commercial xylitol gum three times daily for a minimum of five minutes each time for three months. The placebo group subjects (n = 50) used a commercial sorbitol gum, and the control group subjects (n = 50) did not chew gum. The authors estimated MS load on the dentition using paraffin-stimulated saliva samples. The authors serially diluted the samples, plated them on selective media and incubated them anaerobically; they then enumerated the colonies under a stereomicroscope. Results. MS levels were not significantly different between the three groups at baseline (mean log CFU/mL ± standard deviation: 5.4 ± 0.7, 5.4 ± 0.8, 5.2 ± 0.7, respectively) nor after CHX therapy (2.7 ± 0.8, 3.1 ± 1.1, 3.0 ± 1.1, respectively). After three months of gum chewing, the test group subjects had significantly lower salivary MS levels (3.6 ± 1.2) than did the placebo (4.7 ± 1.2) or control (4.4 ± 1.3) group subjects. Conclusions. Xylitol chewing gum appears to have the ability to prolong the effect of CHX therapy on oral MS. Clinical Implications. Maintaining long-term caries-pathogen suppression is feasible with currently available commercial products and can be expected to result in significant caries inhibition.