UEHARA, S., AKAI, Y., TAKEYAMA, Y., TAKABAYASHI, T., OKAMURA, K. and YAJIMA, A. Pericentric Inversion of Chromosome 9 in Prenatal Diagnosis and Infertility. Tohoku J. Exp. Med., 1992, 166 (4), 417-427-In order to evaluate the relation between pericentric inversion of chromosome 9 (inv(9)) and clinical problems, the characteristics of inv(9) were investigated on the basis of chromosomal analyses of fetuses and infertile couples. The incidence of such inversion in fetuses with parents having an offspring who suffered from various clinical problems was significantly higher than the basic incidence obtained in fetuses karyotyped by reason of advanced maternal age. In the chromosomal examination of the parents whose fetuses were diagnosed as inv(9), it was revealed that either parent might be the carrier. Furthermore, in the inv(9) carrying fetuses, the number of females was significantly greater than that of males. Analysis of infertile couples revealed that the incidence of such inversion in males was significantly higher than the basic incidence mentioned above. Moreover, infertile couples with an inv(9) carrier showed a significantly higher incidence of intrauterine fetal death, compared with infertile couples with a translocation carrier or those in which the etiology was unknown. These results indicate that inv(9) may often cause clinical problems in offspring of the carrier and infertility with unknown mechanisms related to sex.