Cystic fibrosis (CF) and the nonclassical forms of congenital adrenal hyperplasia (NC21OHD) are frequent autosomal recessive diseases. Their frequencies in the Caucasian population are the ones applied to the management of the diseases (1:2500 and 1:1000, respectively). The aim of this study was to learn about the adequacy of these figures to the Spanish population.Study of the recurrent mutation (about 50% of alleles) in each entity, DF508 or Val281Leu, respectively, in 300 anonymous consecutive samples (600 alleles) from the Comunidad de Madrid neonatal screening. PCR specific amplifications of the CFTR and CYP21A2 genes and direct analyses of DF508 and Val281Leu mutations were performed.Allele frequencies of DF508 and Val281Leu were 0.005 and 0.038 (carrier frequencies 1.1% and 7.5%, respectively). Taking into consideration the percentage of these mutations in patient alleles, 48% for CF and for 57% NC21OHD, disease frequencies of about 1:8028 (CF) and 1:230 (NC21OHD) were estimated. The standard error calculated for these data were 0.6% and 1.5%.We have found that CF is less frequent and NC21OHD more frequent in the Spanish population than in other Caucasian populations.