Human endometrium and decidua contain large numbers of different leukocyte populations, the concentration of which fluctuates during the menstrual cycle and pregnancy. There is, for example, a large influx of neutrophils into premenstrual endometrium associated with an increased expression of interleukin (IL)-8 protein, which is chemotactic for neutrophils. Our aim in this study was to localize IL-8 messenger RNA (mRNA) expression in endometrium and decidua using in situ hybridization. In situ hybridization was carried out with a 35S-uridine 5'-triphosphate-labeled riboprobe using standard procedures. Late secretory endometrial and decidual biopsies demonstrated clear perivascular localization of IL-8 mRNA, with additional expression colocalized to activated macrophages. Midluteal endometrium showed minimal IL-8 expression, whereas endometrium obtained from women administered progesterone for 4 days from (LH peak + 8 days), to simulate luteal regression, demonstrated significantly increased localization of IL-8 mRNA, 48 h after withdrawal of progesterone. In conclusion, IL-8 mRNA expression is localized to perivascular cells of late secretory endometrium and decidua.