The aim of the study was to assess the effectiveness of worksite group counselling interventions designed to prevent smoking relapse after abstinence has been achieved following 3 months therapy using group support and/or transdermal nicotine replacement therapy. After 3 months, abstinent subjects were randomly allocated either to a counselling group led by professional psychologists (PG), to a counselling group led by former smokers (SG) or to no intervention group (NG). The 3 and 12 months abstinence were defined, respectively, as a sustained smoking cessation during the last month, and the last 9 months. Complete abstinence was confirmed by expired carbon monoxide and by urine cotinine concentrations. The abstinence rate at 3 months was 35.1%. After 12 months abstinence rates were not statistically different in the PG, the SG and the NG (respectively 57.8, 53.4 and 49.6% of those randomised). In multivariate analyzes, baseline variables associated with 12 months abstinence were non-smoking family, gender (male), lower daily intake of nicotine and better psychological adjustment. Mean weight gain at 3 months in abstinent versus relapsed subjects, was respectively, 4.1 and 2.4 kg. Baseline variables associated with weight gain at 3 months were higher Fagerström score, gender (male) and professional status (blue collar worker). Group support after abstinence has been achieved did not significantly improve the abstinence. This study shows the difficulty of preventing smoking relapse with monthly group counselling. The results indicate the need to investigate further specific programmes focusing on factors such as gender, family, nicotine dependence, psychological and weight concerns/issues which may precipitate relapse.