The case of a 28-year-old woman under wasp venom desensitization having a premature birth in her 24th week of pregnancy 16 days after the last injection is described. To test the hypothesis that a special profile of immune cells in the decidua may trigger abortions, placental and decidual tissue sections were stained with antibodies against T cells (CD3), cytotoxic cells (CD8), natural killer cells (CD56), and mast cells, and an in-situ-hybridization was performed for tumor necrosis factor-α (TNF-α). CD56+ Natural killer cells were the dominating population. In earlier analyses of healthy first trimester decidua the percentage of NK cells and T cells was in a similar range, but the CD8:CD3 ratio was only 2.2% in contrast to 27% in the present case. Mast cells, which are known to be able to secrete abortogenic TNF-α, were only detectable in the decidua (10 cells/mm2) and decidua sections were TNF-α positive. Since SIT induces a shift of the interleukin and functional profile from a Th2 type towards a Th1 type, and pregnancy is dependent on a Th2 pronounced profile, SIT may trigger abortions or immature births. This is supported by the present results and might have happened in this case.