The antenatal sonographic diagnosis of exencephaly in four gestations is 
reported. Exencephaly is an uncommon malformation of the cranium that 
characteristically involves a large, disorganized mass of cerebral tissue. The 
flat bones of the calvaria are absent, leaving the brain mass uncovered. 
Secondarily, anencephaly may develop as a result of prolonged exposure of the 
developing encephalon to amniotic fluid and trauma in utero. As in anencephaly, 
facial structures and the bony base of the calvarium are often preserved in 
exencephaly. Sonographically, the outstanding feature of exencephaly is the 
cerebral mass, with convolutions or "pseudo" sulcal patterns present. These 
findings correlate well with the pathologic examination and define a clinical 
entity that is incompatible with human life.