Presented herein is a randomized prospective study performed to evaluate the efficacy of the addition of lymphocyte immunotherapy (LI) to progesterone (P) therapy (LI/P) for the prevention of spontaneous abortion (SAB) in primary aborters with a history of three SABs. The incidence of intrauterine pregnancies in four cycles was 23 of 35 (65.7%) patients for LI/P vs. 14 of 31 (45.1%) patients treated with progesterone alone. SABs occurred in 6 of 23 (26.0%) Ll/P-treated patients compared to 8 of 14 (57.1%) given progesterone alone. The mean number of previous abortions in both groups was 3.9. The mean age of the LI/P group was 34.1 vs. 33.6 years for the group treated with progesterone alone. These data could be interpreted to show that progesterone therapy and LI independently inhibit SAB or that LI/P acts synergistically to inhibit immune destruction. LI/P therapy was found to be more effective than progesterone therapy alone.