Ultrasonographic examination at 16 weeks 5 days' gestation diagnosed a monoamniotic triplet gestation complicated by thoracopagus conjoined twins. After pregnancy termination, the ultrasonographic diagnoses were confirmed by pathologic examination. In reporting the first ultrasonographically diagnosed monamniotic triplets with conjoined twins, we demonstrate that a systematic evaluation by twin-derived ultrasonographic techniques may permit correct diagnosis of unusual triplet pregnancies.