Prenatal sonographic findings of sirenomelia (or mermaid fetus) were retrospectively reviewed in eleven proven cases. Sonography showed oligohydramnios in all the cases, five (45%) of which had severe oligohydramnios that limited prenatal diagnosis by poor visibility. In five cases (45%), sirenomelia was correctly diagnosed by ultrasound; in the remainder, only bilateral renal agenesis was identified. All eleven fetuses had other associated malformations: congenital heart defects (4), skeletal deformities (10), and abdominal wall defects (4). Death resulted from termination of pregnancy in six cases and stillbirth in three cases. Two newborns died at 24 and 36 hours of neonatal life, respectively. We concluded that some cases of sirenomelia can be detected on prenatal sonograms by demonstration of a single lower extremity, oligohydramnios, and bilateral renal agenesis. Sirenomelia is a lethal condition and can be detectable in the second trimester of pregnancy, allowing for termination of pregnancy.