Abstract. The aim of this study was to assess the effectiveness of a dentifrice containing 0.3% triclosan, 2.0% copolymer and 0.243% sodium fluoride (Colgate Total®) in the prevention of periodontal attachment loss in adolescents. A 3-year, double-blind, randomised, controlled, clinical trial was conducted on 641 adolescents, initially aged 11–13 years. The participants were enrolled from schools in economically deprived areas of Manchester, England, which had a high % of Asian pupils originating from India, Pakistan and Bangladesh. After the baseline examination, which included assessments of pocket depth, attachment loss and subgingival calculus, the adolescents were randomly allocated to use either a triclosan/copolymer or control dentifrice. The control was identical to the test dentifrice with the exception that it did not contain triclosan/copolymer. Participants were re-examined after 18 and 36 months. After 3 years, a total of 239 adolescents remained in the test and 241 in the control group. The prevalence of attachment loss increased from 2% at baseline to 24% after 3 years. The mean increment of attachment loss during the study was 0.025 mm per site in the control group and 0.018 mm per site in the test group. A linear regression model showed a statistically significant difference in attachment loss between the test and control group, in adolescents with high mean pocket depths at baseline. No adverse effects attributable to the test or control dentifrices were observed during the study. This study has demonstrated that unsupervised use of a triclosan/copolymer dentifrice can significantly reduce periodontal attachment loss, particularly in adolescents with high mean pocket depths.