SUMMARY A new dicentric X isochromosome i(X)(pter-*q2102-*pter) of similar size to a normal X is described in a girl with gonadal dysgenesis. In this non-mosaic case with an X short arm duplication, most of the stigmata associated with Turner's syndrome were absent. This structure was compared with that of six i(Xq) and three del(X (1977), Niebuhr and Skovby (1977), and Verlinskaya and Mashkova (1977). Of considerable interest and significance as to their genesis is the proportion of isochromosomes that prove to be dicentric. Centromeric misdivision was proposed as the mechanism of origin of isochromosomes by Darlington (1939), who also predicted the existence of dicentric isochromosomes which he termed isodicentrics (Darlington and Wylie, 1953). This concept was developed in human material (de la Chapelle et al., 1966) and possible infracentromeric breakpoints and dicentric-producing long or short arm breaks were described. Therman et al. (1974) showed that dicentric X structures such as end-toend X fusions fitted the model of Gl chromatid break with G2 sister chromatid reunion, producing a dicentric chromatid at the following anaphase. A second alternative proposed by Therman et al. was a G2 chromatid exchange after homologous pairing of the adjacent type, analogous to the U type exchanges within a bivalent observed in plants (Jones, 1969). Though the existence of Xp isochromosomes has been doubted after the reinterpretation of one as an Xq deletion (de la , it has been suggested (Howell