We report a case of sirenomelia of the symelia dipus type which is associated with complete situs inversus. Sirenomelia is difficult to diagnose prenatally because of a frequently associated oligohydramnios. Symelia dipus type adds to the difficulty of the diagnosis, since the bones of the thighs and legs are fully formed. In fact, a close examination of the cases of sirenomelia previously reported in the literature shows that a prenatal diagnosis was achieved only when the oligohydramnios was no more than moderate and/or the associated malformation was of apus or unipus type. We propose a systematic amnioinfusion when one suspects symelia unipus type. Finally, the pathogenesis of sirenomelia is discussed, as the presence of a complete situs inversus in the case reported here could illuminate the debate concerning the origin of the malformation.