The role of occupational exposure to fiberglass in the development of work related asthma (WRA) was investigated. Information on nasal, eye, and throat symptoms, chest tightness, shortness of breath, and wheezing and their relationship to work was derived from questionnaires given to 216 employees. Information on smoking, occupational history and hay fever, asthma and chronic bronchitis was gathered. Simple spirometry tests were performed and serum samples were taken. Samples of water, slime, and dust were taken from humidifiers used in the facility. Bronchial challenges of four symptomatic subjects were performed in a hospital. Four symptomatic subjects were challenged in a hospital with fumes for 31 or 60 minutes, with glass fiber fly, dressing used in the industry consisting of polyvinyl-acetate or epoxy compound, humidifier antigen, and histamine. A control challenge was performed using an atopic, nonsmoking asthmatic with no fiberglass exposure. About 40 workers had symptoms of WRA. Peak flow recordings of 32 were assessed. Only seven cases of WRA were identified. Bronchial challenge testing did not identify a cause for WRA. The authors conclude that the cause of WRA in the filament glass fiber facility remains unknown.