Clinical and chromosomal studies are reported for 30 female patients selected because they show at least one of the characteristic features of Turner’s syndrome. Exploratory laparotomy has been performed in eight patients, and culdoscopy in two others. Seven patients have an XO sex chromosome constitution, eight have XO/XX mosaicism, two have probable XO/XX/XXX mosai-cism, one has XX1, five have XO/XXi mosaicism, one has a presumptive deletion of the long arm of an X chromosome, two have small structural X or Y chromosome aberrations, one has XO/XY mosaicism and three have an apparently normal XX karyotype. Xg grouping in one XO patient and her parents reveals that the X chromosome may be maternally derived, and similar studies in one X isochromosome-X patient suggest that the isochromosome-X may be paternally derived. Parental age was found to be normal. Sex chromatin findings in the patients with an isochromosome-X, and in a patient with an X-deletion, confirm that the structurally abnormal X forms the sex chromatin body. Karyotype-phenotype correlations in these, and in previously reported patients, support the hypothesis that the usual cause of Turner’s syndrome and its variants is variable deficiency of the X and Y chromosomes, due both to mosaicism and to deletion involving postulated homologous loci. “Streak” gonads are the usual consequence of a large deficiency of any part of the X, but the marked short stature and associated stigmaa of Turner’s syndrome are only observed in cases where the deficiency includes the short arm of the X (including the pairing segment) or part of the Y. The karyotype-phenotype correlations support the concept that a part of the short arm of both X chromosomes is genetically active in human female soma