Introduction: Using spirometry data from workplace screening programs conducted at 23 assorted manufacturing plants typically absent current regulated exposures, we estimate the effect of previous occupational exposure (as reported on a questionnaire at entry into the program) on baseline spirometry (FEV1 FVC, and FEV1/FVC). Methods: 17,878 workers participated in the annual spirometric monitoring programs implemented from 1984 to 2002. At each spirometric testing, questionnaire data were collected on previous occupational exposure to asbestos, sandblasting, mining, cotton dust, and chemical gases and the duration of each exposure. Smoking habits (status and number of cigarettes smoked) were also ascertained. Using the linear regression model, we estimated the effect of each previous occupational exposure (present/absent) and the duration of each exposure on lung function (% predicted), while we adjusted for the effects of smoking status, pack years, plant, and the calendar year. Results: Across all plants, previous occupational exposures to mining (5 yrs or more), chemical gases (10 yrs or more) and cotton dust (10 yrs or more) were associated with significantly lower FEV1% pred. In specific plants, exposures to sandblasting and asbestos were also associated with significantly lower FEV1. Conclusion: The results suggest that the effect of previous occupational exposures should be taken into consideration to obtain unbiased estimates of the effect of current occupational exposure on lung function.