AIM: To evaluate the effectiveness of the Five-Day Plan (FDP) in helping smokers to stop smoking. DESIGN: Randomized controlled trial comparing intervention and control groups. The primary outcome measure was 12 months continuous abstinence verified by expired air carbon monoxide concentration. Secondary outcome measures were self-reported abstinence at end of treatment, at 3 and 6 months. SETTING: Six towns in France. PARTICIPANTS: 228 smokers, recruited by newspaper and radio advertisement, aged 18 years or over and willing to make an attempt to quit smoking. INTERVENTION: The Intervention group (119 participants) received the FDP, which is a behavioural group-based treatment programme that has been in operation in France since 1965. It involves five consecutive evening behavioural therapy sessions. The Control group (109 participants) received a single session discussing the health effects of smoking. FINDINGS: In the Intervention group, 67 participants (56%) quit smoking at the end of the FDP. After three months this number had been reduced to 30 (25%) and to 19 (16%) by the end of one year. In the Control group these numbers were 14 (13%) and 12 (11%), respectively, after three and 12 months. When considering the rate of cessation without lapse after one year a significant difference was observed with a 13% rate in the Intervention group and 3% in the Control group (P = 0.004). CONCLUSIONS: The FDP may be considered as an aid for smokers who want to quit.