The purpose of the present study was to evaluate the oxygen uptake (VO2) response to walking in patients with peripheral arterial occlusive disease (PAOD) at a constant velocity and to determine whether the oxygen uptake observed during constant-intensity exercise in this population is associated with the severity of PAOD. Forty-nine PAOD patients with intermittent claudication were recruited (values are mean +/- SD) [age = 66.4 +/- 8.0 years, weight = 83.3 +/- 16.0 kg, body mass index = 28.8 +/- 5.6, ankle/brachial systolic blood pressure index (ABI) = 0.63 +/- 0.18, VO2peak = 13.02 +/- 2.99 ml/kg/min]. Patients were evaluated during a 2.0 miles/h walk on a treadmill until maximal claudication pain or for a maximum of 20 minutes. On average, patients walked for a duration of 10.3 +/- 5.8 min. Despite the constant absolute intensity during the exercise bout, there was a significant (p < 0.01) 4.0% increase in VO2 from min 3 of exercise (10.58 +/- 2.02 ml/kg/min) to min 5 (11.01 +/- 2.18 ml/kg/min) and a further 4.0% increase from min 5 to the end of exercise (11.46 +/- 2.32 ml/kg/min). Expressed as relative exercise intensity, this represented an increase from 80.8 +/- 18.3% to 87.1 +/- 18.0% of VO2peak from min 3 to the end of exercise. The magnitude of increase in VO2 during the exercise bout was not correlated with resting ABI (r = 0.00, p = 0.68) or post-exercise ABI (r = 0.04, p = 0.73). There was a nonsignificant trend towards a correlation between the time to onset of claudication pain and the magnitude of increase in VO2 (r = 0.25, p = 0.08). Results of this study suggest that an increase in VO2 occurs during constant-velocity walking at a high relative intensity in PAOD patients. This increase in VO2 does not appear to be related to the severity of PAOD.