Fetal upper respiratory tract function was studied in five cases of antenatally diagnosed congenital diaphragmatic hernia and in 16 cases of uncomplicated pregnancy at gestational ages ranging from 27 to 38 weeks. The evaluation of fetal upper respiratory tract function was performed using ultrasonography combined with color-flow and spectral Doppler analysis. In all cases with uncomplicated pregnancy, fetal breathing-related nasal and oropharyngeal fluid flow was seen at the level of the nose. The five cases with congenital diaphragmatic hernia all demonstrated fetal breathing activity by thoracic wall movement. In four of the fetuses, perinasal fluid flow was seen by the Doppler technique. The fetus with no demonstrated perinasal flow during breathing movements died in the early neonatal period and had pulmonary hypoplasia. Observation of the fetal breathing-related nasal and oropharyngeal fluid flow in cases of antenatally diagnosed congenital diaphragmatic hernia provides a rationale to hypothesize that the absence of this phenomenon is a useful marker for prenatal prediction of pulmonary hypoplasia.