AbstractA report is presented of a case of sirenomelia (symmelos). The affected infant was a partner in a monozygous monoamniotic twin pregnancy; the other twin is surviving and normal. In addition to anomalies regularly associated with sirenomelia such as bilateral renal agenesis, blind‐ending colon, severe abnormality of the bony pelvis, absence of both urethral and anal orifices, and medial position of the fibulae, the features of another syndrome were present composed of dextroposition and severe, complex abnormality of the heart, hyperlobation of lungs, and asplenia. Some aspects of this case seemed to be contrary to all experience of sirenomelia, e.g., the presence of both umbilical arteries, the absence of Potter facies, of hypoplasia of lungs, and of amnion nodosum, and the relatively long survival. This group of atypical features (with the exception of the normal number of umbilical arteries) can be explained by the fact that, unlike most other cases of sirenomelia, there was no oligohydramnios. This was so, presumably, because the normal twin maintained an adequate amount of amniotic fluid in the common amniotic sac.