The prenatal ultrasonographic features found in two cases each of the Pena-Shokeir I and the trisomy 18 syndromes are described. All four cases were referred for ultrasonographic evaluation of polyhydramnios, clinically detected in the third trimester. A detailed ultrasonographic study of the craniofacial structures, intrathoracic organs, and limbs showed scalp edema, multiple ankyloses, camptodactyly, rocker-bottom feet, cardiac arrhythmias, and lung hypoplasia. Chromosome analysis showed trisomy 18 in two cases; in the other two cases the diagnosis of the Pena-Shokeir I was confirmed after delivery. The similarity of the two syndromes with respect to ultrasound findings is discussed.