OBJECTIVE This study assessed the prevalence of respiratory manifestations among French pig and dairy farmers and determined the relationship between bronchial reactivity and respiratory manifestations. METHODS The pig farmers included 102 men working more than half-time inside swine confinement buildings. There were 51 male dairy farmers and 81 male referents. The demographic characteristics of the three groups were similar except for more smokers among the referents. Each subject completed a standardized questionnaire. Pulmonary function tests were performed before and after a methacholine challenge (cumulative doses 80, 240, and 560 micrograms). Airborne dust, ammonia, and carbon dioxide were measured inside 28 swine confinement buildings. RESULTS The pig farmers were exposed to a total dust level of 2.41 mg.m-3. The respirable particle concentration was low. The pig and dairy farmers had a significantly higher prevalence of cough and morning phlegm than the referents. Before the methacholine challenge, the dairy farmers had nonsignificantly lower mean lung function values than the other groups. Among the subjects with no history of asthma, nonspecific bronchial hyperreactivity was significantly higher among the pig and dairy farmers than among the referents. There was a fall in the forced expiratory volume in 1 s (FEV 1.0) that was greater than 10% in 6.7% of the referents, 17.9% of the swine workers, and 35.6% of the dairy farmers. This result was unchanged after adjustment for the initial FEV1.0. CONCLUSIONS The prevalence of respiratory symptoms was significantly higher among the pig farmers without base-line lung function impairment. However, both the pig and the dairy farmers had increased bronchial reactivity.