21,013 newborns were screened on the first year of a neonatal screening program on congenital hypothyroidism in the Basque Community. The evaluation of the results showed some points of interest. One of them, would be the high analytical retesting rate needed in the screening of very low-birth weight children. The rate decreased with increasing birth-weight. Forty per cent of the children with a birth-weight lower than 1,000 gr needed retesting. The rate decreased to 7.7% at birth-weights between 1,001 and 1,500 gr, to 2% at birth weights between 1,501 and 2,500 gr and only to 0,42% at birth-weights higher than 2,501 gr. The consequences of these results are discussed.