ObjectiveThis 12-month study in a primary healthcare network aimed to assess the effectiveness of usual smoking cessation advice compared with personalised information about the spirometry results.DesignRandomised, parallel, controlled, multicentre clinical trial.SettingThis study involved 12 primary healthcare centres (Tarragona, Spain).ParticipantsActive smokers aged 35–70 years, without known respiratory disease. Each participant received brief smoking cessation advice along with a spirometry assessment. Participants with normal results were randomised to the intervention group (IG), including detailed spirometry information at baseline and 6-month follow-up or control group (CG), which was simply informed that their spirometry values were within normal parameters.Main outcomeProlonged abstinence (12 months) validated by expired-CO testing.ResultsSpirometry was normal in 571 patients in 571 patients (45.9% male), 286 allocated to IG and 285 to CG. Baseline characteristics were comparable between the groups. Mean age was 49.8 (SD ±7.78) years and mean cumulative smoking exposure was 29.2 (±18.7) pack-years. Prolonged abstinence was 5.6% (16/286) in the IG, compared with 2.1% (6/285) in the CG (p=0.03); the cumulative abstinence curve was favourable in the IG (HR 1.98; 95% CI 1.29 to 3.04).ConclusionsIn active smokers without known respiratory disease, brief advice plus detailed spirometry information doubled prolonged abstinence rates, compared with brief advice alone, in 12-month follow-up, suggesting a more effective intervention to achieve smoking cessation in primary healthcare.Trial registration numberNCT01194596.