Data from a prospective epidemiological study that included 2025 plain cigarette smokers were analysed to investigate the effect of phlegm production of changing to filter cigarettes or quitting smoking. During a 5-year follow-up, 189 subjects quitted smoking, 312 changed to filter cigarettes, while 1524 continued to smoke plain cigarettes. Multiple logistic regression was used to adjust for age, duration of smoking, number of cigarettes smoked, socio-economic status and alcohol consumption. Smokers with morning phlegm at enrollment, who changed to filter cigarettes during the follow-up, had a probability ratio of 1.9 of phlegm production ceasing, compared with smokers who continued to smoke plain cigarettes (P less than 0.05). However, the probability ratio of developing morning phlegm among smokers who changed to filter cigarettes compared to those who continued to smoke plain cigarettes was 1.6 (P less than 0.05). The overall prevalence of morning phlegm at the end of the follow-up was the same in smokers who changed to filter cigarettes as in persistent plain cigarette smokers. The trends for chronic phlegm were similar, although they did not reach statistical significance. The smokers who stopped smoking had a probability ratio of 0.4 of developing morning phlegm (P less than 0.01) and a ratio of 7.7 for ceasing to produce it (P less than 0.001) compared to the smokers who continued to smoke plain cigarettes. Our results suggest that changing from plain to filter cigarettes is associated with a higher frequency of cessation of phlegm production, but offers no protection against the development of phlegm.