Exercise programs are a mainstay of pulmonary rehabilitation for COPD. COPD patients with elevated PCO2 are severely impaired and might benefit from rehabilitation more than other patients. However, there is no systematic data to indicate that hypercapnic COPD patients benefit from intensive rehabilitation. Indeed, in patients with hypercapnia, increased exercise might overtax respiratory muscles, which are weak relative to those of eucapnic patients. To investigate this issue, we reviewed all COPD patients admitted to our pulmonary inpatient program from 1983 to 1986 (n = 317). The program includes multiple daily sessions of upper and lower extremity exercise to tolerance. We assessed admission and discharge pulmonary function tests, arterial blood gases (room air), and functional status. Ambulation distance on a 6-min walk test was used as an objective measure of functional status. Patients were grouped according to the results of their admission room air PCO2. We found that eucapnic patients (n = 197) significantly increased ambulation (admission to discharge) from 409 to 816 feet (p < 0.001). Hypercapnic patients improved as well. Patients with moderate hypercapnia (PCO2, 45 to 54 mm Hg; n = 86) increased their ambulation from 330 to 663 feet (p < 0.0001). Patients with severe hypercapnia (PCO2 > 54 mm Hg; n = 34) increased their ambulation from 336 to 597 feet (p < 0.0001). We found a small but significant improvement in discharge pulmonary function and arterial blood gas results. We conclude that COPD patients with hypercapnia, despite severe ventilatory impairment and weak respiratory muscles, tolerate exercise well and benefit significantly from intensive inpatient pulmonary rehabilitation.