We have prospectively studied the respiratory health of a cohort of grain elevator workers in the Canadian west coast terminal elevators, beginning in 1975 and following them for 6 yr. We have used a "nested" case-control model to identify determinants of the worst trend in FEV1 over this period. The 10% of participants in this category had a mean decline of greater than 100 ml/yr. They were significantly more likely to have had a decline in FEV1 over a work week and to have nonspecific bronchial hyperreactivity. There was a significant relationship between the odds ratio of being a case and the mean level of dust exposure associated with the job and location at the work site; cases were associated with mean total dust levels greater than 5 mg/m3. We found no relationship between the odds ratio of being a case and a number of host factors, such as immediate skin reactivity to common allergens, history of asthma, bronchitis, or hay fever, or presence of respiratory symptoms. We conclude that exposure to grain dust at levels greater than 5 mg/m3 is associated with a serious adverse trend in FEV1.