Dicentric X chromosomes of different derivations were present in 6 out of a total of 42 patients with Turner's syndrome. The most unusual cases were observed in 2 patients having a psu dic(X)(Xpter-Xq21::Xq21-Xpter) in mosaic form who were examined by fluorescence in situ hybridization. Alu-PCR products from hybrid cell lines were used as partial chromosome paints for the p arm of the human X chromosomes. The other 4 patients displayed isodicentric idic(Xq) in mosaic form. One of the patients displayed 9 cell lines originating from the accumulation of isodicentric idic(Xq) chromosomes, along with loss of chromosome material which resulted in fragments of varying size.