Within 3 months of the opening of a salmon-processing plant in the UK, some workers complained of symptoms suggestive of occupational asthma. A survey of all 291 employees identified 24 (8.2%) with occupational asthma. The employees worked near machines which generated respirable aerosols containing salmon-serum proteins. The IgE response to these proteins was associated with occupational asthma (p < 0.001), with increasing severity of symptoms (p < 0.001), and with working distance from the aerosol source (p = 0.037). The main factor which predisposed to IgE-antibody production and asthma was cigarette smoking (p < 0.001), whereas atopy and a previous allergic history did not. The affected employees were reallocated to a low-exposure worksite and factory ventilation was improved. Eleven showed significant clinical and pulmonary function improvement, and continued in employment. Thirteen who still had symptoms were advised to leave, thereafter becoming symptom-free, and regaining normal respiratory function. Early recognition of symptoms and prompt action to reduce aerosol exposure avoided the long-term reduction in pulmonary functions often associated with occupational asthma.