A complex chromosomal mosaic was observed in cultured lymphocytes and skin fibroblasts of a male infant with multiple congenital malformation. The clinical features of the patient overlapped with those of 18p-, 13 trisomy and 18 trisomy syndromes. The karyotype of the modal cell line was 45,XY,t(13;18)(p11.1-2;p11.2), in which the translocation chromosome has one or two functional centromeres; minor cell lines were 46,XY,t(13;18), +13p-; 46,XY,t(13;18), +18p-; 46,XY,r(13), 18p-; 46,XY,13p-,18p-;etc. This chromosomal mosaicism may have been derived from breakage-fusion-bridge (BFB) cycles and centromere inactivation in the early cleavage stage of the embryo.