The sonographic findings in fetal triploidy syndrome include intrauterine growth retardation, hydrocephalus, oligohydramnios, and hydropic changes of the placenta. Ultrasonography can establish the diagnosis only when placental findings coexist with a fetus. Although the majority of triploid conceptions abort spontaneously in the first trimester, occasionally they will progress further, but rarely to term. Six cases are presented in which the diagnosis was suspected by early ultrasound examinations, including one case in which there was an unusually large trophoblastic cyst. Determination of the karyotype is important for the management of a pregnancy with a live fetus, and has implications for genetic counseling of subsequent pregnancies.