Interleukin (IL)-18 acts in synergy with IL-12 to promote development of T helper 1 (Th1) responses. On the other hand, IL-18 alone has the capacity to induce Th2 responses. Here, we have measured IL-18 and IL-12 secretion by non-stimulated peripheral blood mononuclear cells (PBMC) from 17 non-pregnant women, 21 healthy pregnant women, 9 mildly pre-eclamptic patients and 15 severely pre-eclamptic patients. Th1/Th2 ratios in PBMC were determined by flow cytometry. PBMC from healthy pregnant subjects secreted more IL-18 and less IL-12 than non-pregnant women. PBMC from severely pre-eclamptic patients secreted more IL-12 than those from healthy pregnant women, while IL-18 secretion in mildly pre-eclamptic patients resembled that in normal pregnancy. The ratios of IL-18 to IL-12 were significantly higher in healthy pregnant women than non-pregnant women. These ratios were significantly lower in severely pre-eclamptic cases than in normal pregnancy subjects, while these ratios in mild pre-eclampsia resembled those in normal pregnancy. Interestingly, Th1/Th2 ratios were negatively correlated with the ratios of IL-18/IL-12. These results suggest that elevated IL-18 secretion and decreased IL-12 secretion by PBMC may induce Th2 dominance in normal pregnancy, while elevated secretion of both IL-18 and IL-12 by PBMC may cause Th1 dominance in severe pre-eclampsia.