A group of 110 mill workers at a cotton mill on Shijiazhuang, China was investigated using a questionnaire, skin testing, spirometry (FEV(1)) and measurements of bronchial reactivity. They were examined before work started and at 10 weeks and one year thereafter. FEV(1 )decreases over shift were small at 10 weeks and 1 year but slightly higher among persons with a positive skin reaction to cotton extract (skin positive). Bronchial reactivity defined as the average decrease in FEV(1) after 1.25 mg methacholine, increased at 10 weeks. After one year, it remained about the same except for skin positive workers, where it was further increased. Subjective symptoms of chest tightness and cough with phlegm increased progressively at 10 w and 1 year; nasal irritation remained unchanged and dry cough decreased between 10 weeks and 1 year. The results suggest that the airway inflammation caused by cotton dust increases with increasing exposure time and that the changes are more marked among atopic persons.