In the present report we summarize our data on 144 obligate female carriers. The clinical and cytogenetic data are reviewed and discussed. In more than 30% of heterozygotes mental development is borderline to subnormal, and they may also present psychiatric disturbances of which psychotic behaviour is the most frequent. Partial phenotypic expression is present in 28% of the carriers, and is more frequent in the mentally subnormal. Repeated fragile X screening remains negative in more than 50% of the patients. This results in great difficulties in accurate genetic counseling of the individual female at risk. In all females with partial clinical expression fragile X screening is positive. Fragile X carriers have a high fertility. A fourfold increase in twinning is observed and this may be an indication of a disturbed cortico-hypothalamic-hypophyseal axis.