Complete situs inversus is well described in humans and may be associated with defective cilia in Kartagener's syndrome, an autosomal recessive trait. Only half of homozygotes display situs inversus, presumably due to 'chance'. Isomerism sequence or Ivemark syndrome involves major disturbances of organ formation, the patterns of which suggest such individuals have either two right sides or two left sides to the body. Defects in an autosomal and more rarely in an X-linked gene are involved, though again chance factors probably influence expression. Isomerism may occur in one of monozygotic twins, particularly in the right half of conjoined twins. It is likely that the twinning process disturbs laterality in one of the pair. This provides one mechanism to explain the marked excess of heart defects in monozygotic twins. Most of these defects are not, however, associated with overt disturbances of laterality. Since the heart is the organ most sensitive to disturbed situs, one explanation is that the excess reflects a lesser degree of disturbed laterality. This would require a model where there is a gradation from normal situs through isomerism to situs inversus.