We assess the precision and validity of longitudinal spirometry data obtained from workplace monitoring programs with the aim of identifying reliable data for epidemiologic research into the effect of occupational exposure on the development of chronic obstructive pulmonary disease (COPD). Methods: 17,878 workers from 23 assorted manufacturing plants participated in the monitoring programs from 1984 to 2002. Personnel conducting the screening were trained in the ATS guidelines. We assessed temporal changes in the precision of spirometric data over the calendar years 1984-2002 and over subject-specific testing periods (I to 12) using the reliability coefficient G, which measures the relative size of the random error of measurement. Using the general linear mixed model, we analyzed the variation in two spirometry measurements repealed within 12 months of each other on a group of subjects and estimated within- and between, subject variation, from which we then calculated the coefficient G. In this way. we estimated G over follow-up testing periods and calendar years in each plant. To evaluate potential selection biases in the data, we compared the mean FEV1% pred in workers who continued to participate with those who ceased to participate. Results: Across all plants, for the years 1984 to 2002. the respective coefficient G values for FEV1 were: 0.94, no data, 0.92, 0.89, 0.87, 0.90, 0.91, 0.89, 0.88, 0.91, 0.92, 0.92, 0.92, 0.90, 0.91, 0.92, 0.89, 0.93. The above methods allow identification of plants and periods with relatively precise spirometry data and without apparent selective non-participation. Conclusion: The coefficient G and the validity analysis provided a practical tool for identifying relatively precise screening data for epidemiologic research. Monitoring of the coefficient G ill workplace screening programs can assist in maintaining precise spirometry data.