Familial mosaicism has rarely been reported either for autosomes or sex chromosomes. Its recognition poses problems in prognosis, especially in prenatal diagnosis. Three generations of females showed sex chromosomal mosaicism with 3-4 cell lines, the diploid predominant. Phenotypic effect, if any, appeared limited to reduced fertility. The proposita, ascertained prenatally, has grown and developed normally. A dominant gene mechanism most likely accounts for the observations, either autosomal or X-linked. If the mechanism in this family is monogenic, then the gene may not be strictly private considering the frequency of mosaic amniotic fluid cell cultures.