We investigated whether occupational exposure to ozone gassings in pulp mills was associated with increased airflow limitation and decline in lung function. We used both cross-sectional and longitudinal data in our analysis. Bleachery workers potentially exposed to ozone (n=179) from three Swedish pulp mills were studied, as was a comparison group of paper mill workers not exposed to ozone (n=62) from two of the three pulp mills. Initial and follow-up surveys took place in 1996 and 1999, respectively. Participants underwent spirometry testing and provided answers to a questionnaire regarding history of accidental exposure to ozone. gassings. Several- analyses suggested that bleachery workers with a history of four or more ozone gassings (n=25) had obstructive changes when compared to paper mill workers. The group of workers with at least 4 ozone gassings had lower forced expiratory volume in one second (FEV1) and lower ratio of FEV1 over forced vital capacity (FEV1/FVC) in 1999, controlling for potential confounders (age, gender, height, weight, smoking) by linear regression analyses (-0.219 L, p=0.06; -2.69%, p=0.05, respectively). In another regression model controlling for potential confounders, this exposed group also had an additional 17 ml longitudinal decline in FEV1 (L/yr) between 1996 and 1999 in comparison to paper mill workers, although this difference was not statistically significant (p=0.21). In 1999, workers with at least 4 gassings were also more likely to have airflow limitation (i.e., FEV1/FVC < 70%), although the association was not statistically significant at the p<0.05 level (prevalence ratio: 3.72, 95% CI: 0.66-20.94, p=0.14). These findings suggest that obstructive changes in lung function among pulp mill workers are associated with multiple ozone gassings.