PROBLEM: In T lymphocytes, nuclear factor of activated T cells (NF-AT) regulates the induction of cytokine genes upon antigenic stimulation. This study was designed to analyse the relationship between NF-AT and pregnancy. METHOD OF STUDY: With informed consent, peripheral blood cells (PBCs) were obtained from non-pregnant (n = 114), pregnant (n = 604), and puerperal women (n = 52). The expression of NF-AT2 and NF-AT3 mRNAs in PBCs was measured by a quantitative reverse transcriptase-polymerase chain reaction method. RESULTS: In the early pregnancy period, in successful pregnancy, both NF-AT2 and NF-AT3 mRNAs increased significantly, whereas in cases of spontaneous abortion they did not change significantly. After peaking, they decreased gradually and were re-elevated in the ninth and tenth gestational month. In the puerperal period, NF-AT3 mRNAs decreased, but NF-AT2 mRNA showed a comparatively high expression level. CONCLUSION: These findings suggested that in humans NF-AT signal transduction might be involved in alloantigenic recognition and tolerance and play important roles, especially in the early and late period of pregnancy.