The present study was undertaken to examine the respiratory health of a Danish workforce exposed to man-made vitreous fibers (MMVF) during production. Workers with more than 5 yr occupational exposure to MMVF (n = 377) were compared to a group without MMVF exposure (n = 381). Respiratory health was assessed by questionnaire, dynamic spirometry, and measurement of transfer factor. Overall response rate was 63%. A sample of nonresponders was assessed by questionnaire and spirometry. On most spirometric indices the two groups had comparable values. However, a larger proportion (14.5%) of the exposed subjects had an obstructive flow pattern compared with the control subjects (5.3%). Subgroup analyses showed that the elevated risk of airways obstruction associated with exposure was restricted to heavy smokers. Transfer factor and prevalences of symptoms and self-reported disease were similar in the two groups. There is no indication of excess risk of lung fibrosis. However, a number of exposed workers have some degree of airflow obstruction, which cannot be explained by known confounders. An additive or synergistic action between smoking and fiber exposure on airflow obstruction can be speculated.