Incomplete fission of the primordial cell mass is generally believed to be the cause of conjoined twinning. It may also explain the tendency towards increased symmetry in these twins. We report a case of the rostral duplication type of conjoined twins with an absence of symmetry; one twin had the asplenia syndrome, whereas the other twin was normal. Local environmental factors acting upon the organ anlage may explain the asymmetric development in these conjoined twins.