X chromosome mosaicism is usually associated with abnormal sexual development and reproductive performance, such as recurrent spontaneous abortion, primary or secondary amenorrhea, infertility, and premature ovarian failure. However, there is a paucity of literature which discusses these relationships. From July 1988 to December 1992, a total of 105 couples with a history of recurrent spontaneous abortion from a genetic counseling clinic and 61 women with a history of premature ovarian failure followed up in reproductive endocrinology clinics were assembled, and chromosomal analysis of peripheral blood lymphocytes was carried out. X chromosome mosaicism was found in six individuals (2.9%) out of the 105 couples with recurrent spontaneous abortion, and in five (8.2%) out of 61 women with premature ovarian failure. The karyotypes were 45,X/46,XX/47,XXX in five cases, 45,X/46,X in four cases, and 46,XX/47,XXX in two cases. The cases of complex X chromosome mosaicism (45,X/46,XX/47,XXX) presented recurrent spontaneous abortion in four cases and premature ovarian failure in one case. The cases of mosaic Turner's syndrome (45,X/46,XX) presented premature ovarian failure in three cases and recurrent spontaneous abortion in one case. The two cases of mosaic triple-X syndrome (46,XX/47,XXX) presented with premature ovarian failure and recurrent spontaneous abortion, respectively. However, the ratio of mosaic cell lines does not correlate well with the phenotypic manifestations in our cases. Our preliminary data suggest that chromosomal analysis should be done routinely in every couple with recurrent spontaneous abortion and in women with premature ovarian failure. The reproductive performance of X chromosome mosaicism is highly variable and difficult to define.(ABSTRACT TRUNCATED AT 250 WORDS)