A survey was carried out in the processing department of a paper mill, consisting of environmental and personal monitoring, a short questionnaire concerning chronic non-specific lung disease (CNSLD), spirometry, and intradermal tests. Spirometry was performed on Monday, Wednesday, and Friday. Exposures to paper dust regularly exceeded the Dutch Maximal Allowable Concentrations (MACs) for total and respirable dust. There were indications of a qualitatively different exposure to molds inside the mill in comparison with the outdoor air. A comparison between a group of workers exposed to paper dust and a control group showed significantly more persons with positive late and delayed intradermal test reactions in the exposed population. The analysis of the spirometric measurements after correction for differences between the groups in age, height, and smoking habits gave evidence of the existence of lower FEV1, MMEF, MEF50, and MEF25 among exposed workers with a positive immediate intradermal reaction on Friday. The same workers showed a decline in pulmonary function over the week compared with the controls. These changes suggest an obstructive airway reaction with an immunologic mechanism to dust exposure in a paper mill.