M aternal tolerance of the fetal hemiallograft suggests that immunomodulation occurs during gestation. Therefore, recurrent spontaneous abortion (RSA) may represent a failure of the immune changes that maintain pregnancy. We hypothesized that fertile women but not women with RSA may lose their immune responses to recall antigens when pregnant. This phenomenon has been seen in immunosuppressed transplant recipients and is associated with graft survival. Therefore, we evaluated proliferative responses to recall antigens in four groups of women: group 1, nonpregnant fertile women with no history of pregnancy loss and at least one prior healthy pregnancy, n = 13; group 2, nonpregnant women with a history of three or more spontaneous abortions, n = 28; group 3, healthy pregnant women between 6 and 9 wk of gestation without a history of prior pregnancy loss, n = 15; and group 4, pregnant women between 6 and 9 wk of gestation, with a history of RSA, n = 22. Proliferative responses of peripheral blood leukocytes to the recall antigens influenza and tetanus, alloantigens, and phytohemagglutinin were determined prospectively. Positive responses (stimulation index > 3) to recall antigens (a response to either influenza or tetanus was considered positive) were as follows: group 1 (nonpregnant fertile women), 11/13 (85%); group 2 (nonpregnant RSA women), 24/28 (86%); group 3 (pregnant fertile women), 4/15 (27%) (P </= 0.007); and group 4 (pregnant RSA women), 13/22 (59%) (P = 0.032) [corrected]. In group 4, there was 100% fetal survival in the nine women who lost responsiveness to recall antigens; however, in the 13/22 patients who responded to recall antigens, 9/13 (69%) had a repeat spontaneous abortion. These findings suggest that immunosuppression, indirectly measured by proliferation to recall antigens, is necessary for early pregnancy maintenance. Furthermore, this approach may be useful for predicting pregnancy outcome for women with RSA and may provide a useful means for designing and monitoring therapies.