Although cystic fibrosis (CF) is the most common, life-threatening autosomal recessive genetic disorder of the white population, there are often delays in diagnosis and hence initiation of treatment. Advances of the past two decades have made CF screening feasible using routinely collected neonatal blood specimens and determining trypsinogen levels and CF mutations by DNA analyses. Our overall goal is to address the following hypothesis: Early diagnosis of CF through neonatal screening will be medically beneficial without major risks. "Medically beneficial" refers to better nutritional and/or pulmonary status, whereas "risks" include laboratory errors, potential iatrogenic medical sequelae, miscommunication or misunderstanding and adverse psychosocial consequences. Specific aims include assessment of the benefits, risks, costs, quality of life, and cognitive function associated with CF neonatal screening and delineation of the characteristic epidemiologic features of CF. A comprehensive, randomized clinical trial emphasizing early diagnosis as the key variable has been underway since 1985. Nutritional status has been assessed by anthropometric and biochemical methods, and the results have demonstrated significant benefits in the screened group. Answering the important questions about pulmonary outcome will require five more years of follow-up evaluation focused on lung function measures and quantitative chest radiology, including high resolution computerized tomography. If the questions underlying this study are answered favorably, it is likely that neonatal screening using a combination of trypsinogen and DNA tests will become the routine method for identifying new cases of CF and that diagnosis in early infancy will allow prevention of many clinically-significant problems such as malnutrition. If CF neonatal screening is implemented nationally, however, several epidemiologic gaps must be closed, and this will require more precise data on the course of this disease and determination of risk factors for pulmonary infections with Pseudomonas aeruginosa. This project will generate that important information, as well as essential data on the quality of life and cognitive function of children with CF who experience early or delayed diagnosis. We will also clarify the risks of screening and delineate for the first time the costs of diagnosis and treatment of CF throughout childhood as well as the cost-effectiveness of screening.