Assessment of nutritional status is important in cystic fibrosis (CF). We performed body composition analysis by dual-energy x-ray absorptiometry (DXA) in 12 adults with CF, 7 of them with malnutrition as defined by percentage of ideal body weight (IBW) lower than 90. The DXA allowed for determination of bone body mass (BBM), lean body mass (LBM), and fat body mass (FBM). A deficit in BBM and LBM was found to be present in all seven patients with malnutrition when compared with a group of young normal adults. By contrast, deficit in FBM was inconstant. Deficits in bone and lean were associated with each other. They did not depend on the severity of steatorrhea, but were highly correlated to ventilatory impairment. In conclusion, DXA provides new and potentially useful information on the nutritional status of these patients. In addition, it confirms the close association of malnutrition and lung disease in CF.