The pulmonary response to simulated industrial respirator wear during exercise was studied in humans with restrictive lung disease. The cohort consisted of five female patients with restrictive lung disease, mean age 32.8 years. The comparisons were 12 healthy females, mean age 35.2 years. Five comparisons and three patients had a history of smoking. Four of the patients had a diagnosis of sarcoidosis and one was thought to have oxygen toxicity associated with an episode of adult respiratory distress syndrome. Spirometric tests were administered to the subjects while they exercised on a treadmill. The subjects performed 15 second maximal voluntary ventilation maneuvers with no added resistance or with a 5 centimeter water per liter per second (cm H2O/second) inspiratory and 1.5cm H2O/second expiratory resistance to simulate a respirator. Respiratory rate, minute ventilation (VE), tidal volume (VT), pulmonary diffusing capacity for carbon-monoxide, end/tidal carbon- dioxide pressure (PET/CO2), respiratory exchange ratio (RE), and mouth pressure swing (po) were also evaluated. The comparisons showed decreases in respiration rate, VE, and RE, and increases in VT, PET/CO2, and po in response to the added resistance. The patients under the condition of no added resistance had a respiration rate twice that of the comparisons and a VT 33.3 percent smaller than the comparisons. VE was higher and PET/CO2 was slightly decreased, relative to the comparisons. In response to the added resistance, the patients showed changes similar to those of the comparisons, except for a larger increase in po. The authors suggest that the stress caused by added resistance to patients with restrictive lung disease is minor compared with that of mild exercise.