In a randomized, double-blinded study, patients with chronic obstructive pulmonary disease and hypercapnia were fed low, moderate, and high carbohydrate diets to determine the effect on metabolic and ventilatory values. The low carbohydrate diet consisted of 28% carbohydrate calories and 55% fat calories and resulted in significantly lower production of CO2 (p less than 0.002), respiratory quotient (p less than 0.001), and arterial Pco2 (p less than 0.05). At the end of the 15-day study, both the forced vital capacity (p less than 0.05) and the forced expiratory volume in 1 second (p less than 0.05) had improved by 22% over baseline values. Total calories given surpassed daily caloric requirements. This approach, together with a low carbohydrate, high fat mixture, may be beneficial for such patients.