Relationships were investigated between chronic bronchitis and plant dust exposure assessed by quantification of barn threshing and cattle foddering and chronic bronchitis and acute respiratory syndromes after plant mold dust exposure. Two groups of male dairy farmers were studied in the Doubs region, France: 197 with chronic bronchitis and 163 control subjects without chronic bronchitis. There was no relationship between chronic bronchitis and exposure. Thirty-three chronic bronchitis patients had semidelayed respiratory syndromes (SDRS) vs two control subjects. Twenty-seven times out of 33 (16 of 17 in the nonsmokers), the SDRS preceded chronic bronchitis. Past history of acute respiratory syndromes during barn threshing (RSBT) was more frequent in chronic bronchitis; RSBT always preceded chronic bronchitis. It is concluded that host factors are important in chronic bronchitis and that acute effects after exposure may be predisposing factors to chronic bronchitis.