The centric fusion, or Robertsonian, type of translocation has been considered as a fusion of the entire long arms of two acrocentric chromosomes preceded by breakage of the centric hetcrochromatin. Reunion of the long arms is considered to include the centromeres, only one of which is visible by conventional staining methods. Prior to the development of the current banding techniques in human chromosome identification, the possibility that a centric fusion-like translocation might be the result of a reciprocal translocation was merely speculative. With the various banding techniques, we have identified two different types of 13/13 “centric fusion’Mike translocation chromosomes with trisomy 13, i.e., one case of reciprocal translocation, 46, XX,–13,+t(13; 13)(p 12;ql 3), and one true case of centric fusion translocation, 46, XY–13, + t(l3;l3)(pl2;pl2). In the first case, the unequal translocation of two No. 13 chromosomes was evident with the Q- and G-banding patterns; this patient also had a milder clinical manifestation of trisomy 13, since she was only partially trisomie for the long arm of No. 13. In the second case, the Q- and G-banding patterns revealed a symmetrical involvement of two No. 13 chromosomes in the translocation, and the C- and Q-banding techniques showed two distinct centromeres in the translocation chromosome. The latter patient demonstrated the typical trisomy 13 syndrome. The example of these two cases indicates that, with the current banding techniques, the true incidence of reciprocal translocations among so-called centric fusion types of translocations can be more accurately estimated.