A six-month, double-blind parallel clinical study was conducted to evaluate the efficacy on existing plaque and gingivitis of a pre-brush mouthrinse (Colgate Plax, Thailand) containing 0.03% triclosan and 0.13% of a polyvinylmethyl ether/maleic acid (PVM/MA) copolymer in the absence of fluoride, as compared to a matching placebo rinse. A total of 121 subjects were stratified into two balanced groups according to baseline Quigley-Hein Plaque Index scores and Loë-Silness Gingivitis Index scores. Each group was randomly assigned to the use of either the triclosan/copolymer pre-brush rinse or the placebo pre-brush rinse. No subjects received an oral prophylaxis. They were instructed to rinse twice daily for 60 seconds using the provided fluoride dentifrice and soft-bristled toothbrush. After six months' use of their assigned mouthrinse, 118 subjects who completed the study were evaluated for plaque index score and gingivitis index score using the same scoring procedure. After six months, the triclosan/copolymer mouthrinse group provided a 35.48% reduction of plaque (p<0.001) and an 18.82% reduction of gingivitis (p<0.001), as compared to the placebo mouthrinse group. The reduction of the triclosan/copolymer group was most pronounced in the severity manifestation of plaque and gingivitis (p<0.001). The results indicate that twice daily use of a pre-brush mouthrinse containing triclosan/copolymer in the absence of fluoride significantly reduces pre-existing plaque and gingivitis as compared to the placebo rinse.