The effect of intermittent smoking on pulmonary function was assessed among participants in the Lung Health Study, 5887 adult smokers with evidence of early chronic obstructive pulmonary disease (COPD), followed up for 5 years. The mean annual rate of loss in FEV1% of predicted after year 1 was smallest for those who quit at some point during the first year of the study and stayed quit (-0.33%/year, +/-0.05%), intermediate for those who smoked intermittently during the study (-0.58%/year, +/-0.05%) and greatest for those who continued to smoke throughout the study (-1.18%/year, +/-0.03%). Surprisingly, those who made several attempts to quit smoking had less loss of lung function at comparable cumulative doses of cigarettes than those who continued to smoke. Quitting smoking for an interval followed by relapse to smoking appeared to provide a measurable and lasting benefit in comparison to continuous smoking. In this early COPD population, not only quitting smoking but attempts to quit smoking can prevent some loss of lung function. These results provide some encouragement to exsmokers who relapse on their way to complete cessation.