The effects of added resistance to breathing during exercise were studied in patients with obstructive lung disease. The study group consisted of 20 male normal volunteers and two patients with obstructive lung disease. Nineteen patients had mild obstructive disease and ten had moderate obstruction. The mean ages of the comparisons and patients with mild and moderate obstruction were 37.2, 38.5, and 42.3 years, respectively. The subjects performed progressive submaximal treadmill exercise while breathing through a mask to which no resistance, 3.5 centimeters of water per liter per second (cm/l/s) inspiratory resistance and 1.5cm/l/s expiratory resistance (R1), or 5cm/l/s inspiratory and 1.5cm/l/s expiratory resistance (R2) were added. Cardiopulmonary parameters such as respiration rate, tidal volume (VT), minute ventilation (VE), end/tidal partial pressure of carbon-dioxide (PET), respiratory exchange rate (RR), heart rate, and mouth pressure swing (poral) were determined at an oxygen consumption rate of 1.5 liters per minute. With no added resistance, the cardiopulmonary parameters were similar for all subjects. Normal subjects and those with mild obstruction had reduced RR, VE, and respiration rate and increased VT and poral with R1 and R2, compared to the case when no resistance was added. Heart rate was not affected. Added resistance in the subjects with moderate obstruction caused similar changes, but only the change in VE was significant. When analyzed separately using a t-statistic, none of the changes occurring in the subjects with obstructive lung disease were significantly different from those of the comparisons, except for a small increase in PET seen in subjects with moderate obstruction. The authors conclude that cardiopulmonary responses to short term submaximal exercise with small added resistance do not differ significantly between subjects with mild or moderate obstruction and normal subjects.