This study considers the birth defects (BD) observed from 1987 to 1992 in 35/811 newborns with congenital hypothyroidism (CH) diagnosed by neonatal screening and included in the National Register. The BD incidence was higher than in the general population (4.3 vs 2.5-3%) and especially the one of the congenital heart diseases, (CHD) (2.1 vs 0.3-0.8%). Furthermore the CHD were more frequently observed in females than in males (M/F = 1/4.7). These results seem not to be casual but the reasons remain unknown. The most frequent CHD observed were the septal defects and the pulmonary stenosis. Further are presented and discussed the main findings of 2 groups of CH patients (with and without BD). These results are a good instance of the National Register applications, also for less known aspects of the CH like the concomitant BD.