A prenatally diagnosed case of sirenomelia with polydactyly and vestigial tail. We are presenting a prenatally diagnosed case with sirenomelia, vestigial tail and polydactyly. A 30-year-old woman at 16 weeks of gestation with dichorionic twins was admitted to the hospital. Prenatal ultrasound demonstrated fusion of the lower limbs in one member and normal femurs, tibias and fibulas, and normal vertebras in the second twin, suggesting the diagnosis of sirenomelia. The twins were delivered vaginally at 35 weeks after spontaneous onset of labor. The affected newborn died after 24 hours and postnatal examination revealed unseparated lower limbs with extreme retroversion, bilateral pes equinus, unilateral postaxial polydactyly, a vestigial tail on the sacral region, a large and wide penis and anal atresia. There is only one previous report of sirenomelia with vestigial tail in the literature. However, a large, wide penis and polydactyly have not been reported before in association with this anomaly.