Epidemiological data provide evidence that disease activity of T cell-mediated, organ-specific autoimmune diseases is reduced during pregnancy. Although there are several experimental animal studies on the effect of pregnancy on the immune system, the situation in humans is less clear. We therefore performed a prospective analysis of cytokine mRNA expression in whole blood by a new on-line reverse transcriptase-polymerase chain reaction technique and of serum hormone levels during pregnancy in healthy women. The control group included age-matched non-pregnant healthy women. Quantitativecytokine mRNA expression revealed significantly reduced IL-18, interferon-gamma (IFN-gamma), and IL-2 mRNA levels in the first and second trimester in pregnancy compared with non-pregnant women. No difference between groups was detected for tumour necrosis factor-alpha (TNF-alpha) mRNA. IL-4 and IL-10 mRNA were detected at low levels in only 20% of pregnant women and were reduced to a statistically significant extent in the second and third trimester compared with the control group. Changes in IL-18 mRNA expression correlated inversely with serum values for human choriogonadotropin (HCG) and IL-10 serum levels correlated with increases in serum 17beta-oestradiol levels. These data indicate immunomodulatory effects of pregnancy at the cytokine level which may be related to the variations in the clinical course of organ-specific, T cell-mediated autoimmune diseases during pregnancy.