We studied 47 patients with a 45,X/46,X,r(X) karyotype to identify phenotypic differences between these patients and 45,X patients, and to determine whether these differences could be explained by the status of genes within the ring. Only 2 patients had the ‘severe’ r(X) phenotype, and both were consistent with this resulting from functional disomy of genes normally subject to X inactivation. A further 7 patients also carried active rings but these patients did not have a more severe phenotype than those whose rings were inactivated, probably because their rings were smaller and did not contain the (as yet unidentified) genes whose functional disomy is particularly damaging. Patients with a r(X) did not show clear physical differences when compared with a 45,X series, except for a possible reduction in the frequency of oedema in those whose r(X) had an Xq breakpoint distal to DXS128E, at Xq13.2. Thus some protection from oedema may be provided by the presence of two copies of Xq13.2.