A 45,XX,5–,13–, dic+ karyotype, verified by autoradiography and fluorescence studies, was found in a nine-year-old patient with typical clinical features of the cri-du-chat syndrome. A dicentric translocation chromosome was present in 34% of about 250 lymphocyte or fibroblast metaphases examined. Twisting in the intercentromeric region, leading to criss-cross separations, might explain the presence in 4% of the cells of a 46, XX,5p– karyotype. Other metaphases showed only one centromere on the translocation chromosome, which resembled a No. 1 in 49% of the cells and a No. 2 in 10%. Fluorescence studies indicated the presence of a centromere from a No. 5 and from a No. 13, respectively, on these monocentric chromosomes, and possible mechanisms leading to these structural alterations are discussed. Translocation chromosomes differing from these main types were seen in 3% of the metaphases. A fragile site on the long arm of one chromosome No. 16 was found in the father, a paternal aunt, and her son.