A new classification of cardiovascular system in conjoined twins is introduced. A special effort has been made to analyze the degree of fusion and symmetry of hearts and great vessels, based on 14 pairs of conjoined twins. The degree of cardiovascular union of the twins is classified into five types. Cases with no vascular union in cardiac, aortic, and inferior vena caval levels were grouped into type I (four cases). Cases with separate hearts and union between aortas or inferior venae cavae were grouped into type II (three cases). Cardiac fusion at the atrial level was grouped into type III (three cases). All of the three cases showed fusion between right atria (subtype IIIa). The theoretical fusion between the left atria or between the left and right atria is put into subtype IIIb. The type IV represents fusion of both atria and ventricles regardless of the number of the chambers (three cases). Type V represents single heart in one of the twins (one case). The external morphologic type and the situs of each twin pair were closely related to the cardiac abnormalities. Dicephalus and thoracopagus were more likely to be associated with abnormal situs and complex cardiac fusion. Abnormal situs was seen in seven out of 14 cases. Three cases with polysplenia and a case with asplenia showed more complex cardiac abnormalities than those with normal situs or situs inversus.