To analyze the efficiency of the method of neonatal screening for cystic fibrosis (CF) used in Castille and Leon (Spain), which is carried out with blood from Guthrie spots.A total of 36,086 newborns were studied from January 1999 to June 2001. Immunoreactive trypsinogen (IRT) was quantified in all samples and genetic study covering 87.5 % of mutations in the CFTR gene was carried out when IRT levels were > 60 ng/mL. The sweat test was performed in all children in whom at least one mutation was detected.IRT values of > 60 ng/mL were found in 285 children (0.79 %). Of these, eight children (2.8 %) were diagnosed with CF and a further 11 children (3.9 %) with a negative sweat test were found to have one mutation and were thus classified as healthy carriers. To date, no false negatives have been detected.The two-stage screening method fulfills the required criteria. Its sensitivity is 98.5 % and the basic model can be used in other regions although genetic screening should be optimized by pilot programs to identify the local spectrum of CFTR mutations.