In the present study the presence and distribution of cellular adhesion molecules involved in leukocyte binding were investigated in human endometrium. Endometrial biopsies (n = 45) were collected from women at all stages of normal menstrual cycles. Consecutive cryostat sections of endometrium were immunostained with monoclonal antibodies to intercellular adhesion molecule-1 (ICAM-1) and platelet endothelial cell adhesion molecule (PECAM) and haematoxylin and eosin. Primary antibody binding was visualized using a streptavidin-biotin system. Strong staining for PECAM was observed in endothelial cells of all vessel types and in focal areas of stroma including single cells, small clusters and larger aggregates of cells. At menstruation, however, almost the entire stroma stained for PECAM which was temporally related to a massive influx of leukocytes. ICAM-1 staining, which was consistently less intense than PECAM staining, was detected in vascular endothelial cells during the cycle, reaching a peak at menstruation. Unlike PECAM, ICAM-1 staining did not occur consistently across all vessel types. Stromal staining for ICAM-1 was rare except at menstruation, when almost the entire stroma showed positive staining for ICAM-1. No glandular or luminal epithelial staining was detected for either PECAM or ICAM-1. This study demonstrates that PECAM and ICAM-1 are expressed on endothelial cells of veins, arterioles and capillaries, and stromal cells within human endometrium.