BACKGROUND: Little is known about the factors associated with CT-quantified emphysema progression in heavy smokers. The objective of this study was to investigate the effect of length of smoking cessation and clinical / demographical factors on the rate of emphysema progression and FEV1-decline in male heavy smokers. METHODS: 3,670 male smokers with mean (SD) 40.8 (17.9) packyears underwent chest CT scans and pulmonary function tests at baseline and after 1 and 3 years follow-up. Smoking status (quitted >=5, >=1-<5, <1 years or current smoker) was noted. Rate of progression of emphysema and FEV1-decline after follow-up were assessed by analysis of variance adjusting for age, height, baseline pulmonary function and emphysema severity, packyears, years in study and respiratory symptoms. The quitted >=5 group was used as reference. RESULTS: Median (Q1-Q3) emphysema severity,<-950 HU, was 8.8 (5.1 -- 14.1) and mean (SD) FEV1 was 3.4 (0.73) L or 98.5 (18.5) \% of predicted. The group quitted '>5 years' showed significantly lower rates of progression of emphysema compared to current smokers, 1.07\% and 1.12\% per year, respectively (p<0.001). Current smokers had a yearly FEV1-decline of 69 ml, while subjects quit smoking >5 years had a yearly decline of 57.5 ml (p<0.001). CONCLUSION: Quit smoking >5 years significantly slows the rate of emphysema progression and lung function decline.Trial registration: Registered at www.trialregister.nl with trial number ISRCTN63545820.