Asplenia syndrome is one of the heterotaxy syndromes, which many familial and animal studies suggest are caused by the loss of adequate genetic control of normal left-right asymmetry development. Moreover, there has not been any environmental factor documented to cause these syndromes. Asplenia syndrome occurring in a pair of monozygotic twins is reported. In view of the negative family history, a new germline mutant gene might be the aetiology of our patients. Both twins are associated with some degree of discordant complex heart defects within the context of a high degree of "mirror-image" of the unpaired thoracoabdominal organs. CONCLUSION: This report implies that sporadic asplenia syndrome might associate with new mutations and further genetic study may be indicated. These monozygotic twins' discordant phenotypes imply that some unidentified factors play an important role in their ultimate development of the same genetically determined abnormalities.