The study will include about 15 patients with COPD recruited at three pulmonology centers based on two selection criteria: COPD stage II or III or IV according to GOLD criteria and hemoglobin level less than 12 g/dL. Effort capacity will be evaluated chiefly by performing a 6-minute walking test and by measuring VO2 at maximum exertion on a bicycle ergometer with increasing loads. Darbepoetin alfa will be given for 2 months. If the hemoglobin increase is less than 1 g/dL after 1 month, the dosage will be increased by 25%.