Implementing effective prevention programmes is critical to prevent tobacco use. Objectives. This study aims to assess the long-term effectiveness of the "SmokeOut-I" programme in the smoking prevention of school-aged adolescents, by gender. Methodology. A quasi-experimental study was conducted including a pre-test and a post-test, as well as a experimental group (N = 163) and a control group (N = 148) of adolescents attending the 6th grade at Braga schools in 2014/2015. A follow-up was conducted three years later. A self-report questionnaire was administered to both groups before and after the programme implementation, and at the follow-up. The programme was only administered to the experimental group. Results. The programme has shown long-term effectiveness in preventing smoking experimentation, and in enhancing smoking refusal skills. There was a lower prevalence of tobacco use and intention to smoke in the experimental group than in the control group, although not statistically significant. It was found that the experimental group maintained adequate knowledge about smoking over time. Conclusions. The positive effects on health and on the environment justify the broad implementation of this programme, which implies the involvement and commitment of governmental institutions.