A study of changes in peak flow rate as a measure of pulmonary dysfunction in swine confinement workers was conducted. The cohort consisted of 24 swine confinement workers, 22 males, mean age 42.17 years, with work related respiratory symptoms selected from among the participants in an ongoing longitudinal study of respiratory health in swine confinement operators in eastern Iowa. The comparisons consisted of 21 swine confinement workers, 18 males, mean age 41.95 years, without any respiratory symptoms and 25 nonconfinement workers, 23 males, mean age 40.28 years, without any symptoms from nearby farms (neighborhood controls). The subjects were given mini Wright peak flow meters. They recorded peak expiratory flow rates (PEFRs) five times daily for 7 days: immediately after waking, after performing chores, before lunch, before dinner, and before going to bed. The actual times that the measurements were made were recorded. The actual hours of the day that each measurement was made were similar between the three groups; the symptomatic swine confinement workers, asymptomatic swine confinement workers, and neighborhood controls reported their first readings between 0535 and 0920, 0440 and 0940, and 0530 and 0920 hours, respectively. The symptomatic swine confinement workers had consistently lower PEFRs than the two comparison groups. The PEFRs of the two comparison groups did not differ significantly from each other. The PEFRs of all three groups tended to increase during the day. The increases were smaller in the cohort than in the two comparison groups. The authors conclude that swine confinement workers with respiratory symptoms have consistently lower PEFRs than workers without symptoms, suggesting the presence of airway disease in the symptomatic workers. Peak flow meters appear to be a useful indicator of potential airway injury.