The Lung Health Study is a multicenter randomized clinical trial. Participants are smokers aged 35-60 with spirometric evidence of moderate lung function impairment. The objective of the trial is to determine whether a program of smoking intervention and use of an inhaled bronchodilator can slow the rate of decline in pulmonary function over a 5-year follow-up period. This paper describes the background, design, sample size (approximately 6000 participants), and power estimates for the trial, as well as the treatment program and the rationale for the choice of inhaled bronchodilator. Plans for analysis of changes in pulmonary function parameters and for analysis of participants' survival and smoking-related morbidity are also discussed.