Two cystic fibrosis (CF) patients with severe pulmonary disease and malnutrition were followed during a course of long-term home total parenteral nutrition (TPN), which included iv fat emulsion. They gained 7 and 12 kg in body weight, respectively, and their ability to participate in daily activities increased. Progressive exercise testing before and during TPN showed a marked increase in maximal work load: 100% and 30%, respectively. At any given work load, oxygen uptake was increased while the respiratory quotient remained lower after the course of TPN. We believe that the lower respiratory quotient and greater oxygen consumption during exercise were due to a reduction in anaerobic metabolism after TPN.