Sirenomelia is extremely rare congenital malformation that characterized by the complete or incomplete fusion of the low extremities and associated by the various organs’ disorders, such as urinary system, which determined the poor outcome in these infants that died during antenatal or early neonatal period. Prenatal ultrasonography, especially during first-second trimester, permits to establish the presence of sirenomelia. Due to the rarity of this malformation, the own observation of first case of sirenomelia in one infant from twins is presented, which borne in Ukraine. Diagnosis of sirenomelia was established at the planned ultrasonographic screening of multigravida mother. After the birth, neonate with sirenomelia was transferred at the intensive care unit of Lviv regional children’s clinical hospital «OXMATDYT». At the first examination, the throughout fusion of lower extremities, the absent of one kidney, bladder, and external genitalia was revealed. With increasing signs of renal and respiratory failure baby died at 9 days after birth. Conclusions. Sirenomelia is extremely rare congenital malformation that associated with the high mortality during early neonatal period due to the presence of underlying congenital malformations, such as urinary system. Ultrasonography during the prenatal period is the screening methods for the diagnosis of this malformation. No conflict of interest was declared by the authors. Key words: sirenomelia, diagnostic, twins.