Immunological dysfunction has been proposed to explain the etiology of recurrent pregnancy loss (RPL). The immunological environment differs between the decidua basalis and decidua parietalis, and also between RPL cases with normal fetal chromosomes and those with abnormal fetal chromosomes. The problem with analyzing decidual tissues from spontaneous abortions is that cause versus effect phenomena are difficult to distinguish. Recent data show that the immune system in a late-stage miscarriage is completely different from that in an early-stage miscarriage. If immunocompetent cells can cause RPL, the immunological environment may be a causative factor, especially in an early-stage miscarriage, at the decidua basalis, and/or in cases of RPL with a normal embryo. Careful examination of the immune system at the decidua basalis in an early-stage miscarriage in RPL cases with normal fetal chromosomes may reveal useful information. This paper aimed at finding a cause of RPL by analyzing the balance of the immune system between T cells and NK cells in an early-stage miscarriage.