The Lung Health Study is a randomized clinical trial designed to determine whether a smoking intervention program and prescription of an inhaled bronchodilator can reduce the annual rate of decline of FEV1 (FEV1 slope) in cigarette smokers with mild to moderate airflow limitation. During the recruitment period, spirometry measurements were obtained during three screening visits from 5,887 cigarette smokers 35 to 60 yr of age. To improve the statistical power of the study to detect significant differences in the FEV1 slopes among study groups, the spirometry testing protocol carefully controls for multiple factors that are known to increase intraindividual variability of FEV1 and FVC results. These factors include participant preparation, maneuver acceptability and reproducibility, technician training and performance monitoring, equipment design and calibration, and result processing. The objective of this report is to describe the success of the spirometry quality control program in minimizing the short-term intraindividual FEV1 variability. The mean difference between the FEV1 values obtained at the second and third screening visits (mean 25 days apart) was +5.7 ml in women and ™14.7 ml in men, with a mean intraindividual variability of 119 ml in women and 162 ml in men, with a coefficient of variation of 5.8% for both. During only 2.1% of the test sessions were participants unable to produce three acceptable FVC maneuvers with the two best FEV1 values matching within 5% or 100 ml. Institution of site visits and technician performance monitoring was associated with improved test session quality grades and maintenance of high quality grades thereafter.