A significant proportion of recurrent pregnancy loss (RPL) is associated with immune etiologies. The immunological environment is different between decidua basalis and decidua parietalis, and also different between RPL cases with normal fetal chromosomes and those with abnormal fetal chromosomes. Recent data show that the immune system in a late-stage abortion is completely different from that in an early-stage abortion. If immunocompetent cells can cause RPL, the immunological environment may be a causative factor, especially in an early-stage abortion, and/or at decidua basalis and/or in the cases of RPL with a normal embryo. Careful examination of the immune system at the decidua basalis in an early-stage abortion in RPL cases with normal fetal chromosome may reveal useful information.