According to the 1995 CF Foundation patient registry data, 84 percent of mortality in US patients was due to cardiopulmonary causes and the most common reason for hospitalization was an exacerbation of pulmonary disease. Good nutritional and growth status are associated with longer survival and with improvement or stabilization of pulmonary function in malnourished patients who have been renourished. Yet in 1995, 20 percent of patients with CF remain less than 5th percentile for height and 25 percent were less than 5th percentile for weight, indicating significant malnutrition and growth abnormalities. There are no studies of sufficient sample size which prospectively evaluate the effect of sustaining good nutritional status on progression of pulmonary disease in a randomized, long-term protocol. Our objective is to facilitate normal growth and body composition in a group of children with CF and determine if this slows the progression of CF pulmonary disease over a two year period. Ninety children, ages 6 to 8 years will be recruited from four clincial research oriented CF Centers (see below for Performance sites) to participate in a randomized, Nutrition Intervention versus Usual Care protocol. The intervention and data collection activities will be conducted by the CHOP-based research team, either at the CHOP Nutrition and Growth Laboratory, the CHOP Clinical Research Center, or the local CF Center, to ensure the uniformity of the program, accuracy of the data and convenience for the families. Subjects in the Nutrition Intervention group will receive a seven week program for parents and children. The program goals are to improve nutrition and CF knowledge, food-related behavioral management, fat intake, total caloric intake and achieve normal growth and body composition. Subjects in the Usual Care group will receive usual nutritional care based upon current practices of their Centers. Subjects will be assessed over two years (baseline, 12,24 months at CHOP;6 and 18 months at local sites) for measurements which include: pulmonary function, nutritional status (growth, body composition, skeletal maturation, blood values), energy balance (resting, total energy expenditure, stool calories, physical activity), food intake and disease severity (NIH, Schwachman, Brasfield scores, days sick from school, days in hospital, microbiology). We hypothesize that children with better energy balance will have improved growth as indicated by height and weight z scores and will have better pulmonary function as indicated by FEV1. These data will provide the scientific basis for early and effective nutritional intervention to promote normal growth and body composition as standard CF care.