The respiratory system and nutrition are linked. Obesity is sometimes seen in chronic obstructive pulmonary disease (COPD), but its prevalence, the morbidity and mortality induced by it are not known. In addition, the prevalence of malnutrition is high in COPD and the more severe the COPD is, the higher percentage of malnutrition is present. Emphysematous patients are more frequently undernourished than those suffering from chronic bronchitis. Malnutrition is the consequence of the hypermetabolism induced by the higher cost of breathing in emphysema. The survival rate of these patients is negatively affected by malnutrition. A careful assessment of nutritional status must be performed in all COPD patients, especially during an episode of acute respiratory failure. When signs of malnutrition are present, a nutritional intervention should be initiated rapidly. An amount of calories sufficient to meet the energy expenditure increased by the disease must be given. Excessive intake may overstress the respiratory system whose functional reserve is limited in COPD. The diet must include a well balanced percentage of fat, carbohydrates and proteins. Preservation of the fat-free mass is the minimum goal to reach in acute respiratory failure. After the resolution of the acute phase, a gain of weight should be attempted within a rehabilitation program.