We tested the cognitive abilities and educational attainments of 47 patients with a ring X chromosome, to evaluate the extent to which these variables correlated with failure of r(X) inactivation and with mosaicism. We found possession of a r(X) chromosome was associated with an increased risk of significant learning difficulties, and with associated behavioural maladjustment, compared with 45,X Turner females. Nearly a third had been educated outside mainstream schools. The proportion of cells in peripheral blood containing an inactivated r(X) chromosome was negatively correlated with nonverbal IQ. The parental origin of the normal chromosome did not appear to affect adjustment or abilities. In a minority of r(X) cases associated with mental retardation, there had been a failure to inactivate the ring, due to loss of the XIST locus. However, failure of X-inactivation was not necessarily associated with a severe phenotype. The degree of impairment in IQ depended on the size of the active ring, and hence was proportionate to the number of (as yet unidentified) genes whose functional disomy affected brain development and functioning.