Different strategies are available for studying the effectiveness of newborn screening for cystic fibrosis (CF). Although observational studies suggest clinical benefits, their results are difficult to interpret because of the potential for several methodological problems. Randomised studies show that age at time of diagnosis is an important factor in the nutritional status of pancreatically insufficient patients and that a delayed diagnosis increases the risk of malnutrition in childhood. Effects on lung disease are more controversial. Advantages other than better clinical outcome or survival may be obtained by the implementation of CF neonatal screening, mainly the opportunity for presymptomatic trials and treatments and more informed reproductive choices for parents and relatives. Disadvantages of neonatal screening for CF include the detection of heterozygotes and the discovery of mild forms of disease.