Endoglin is a glycoprotein which is predominantly expressed on endothelial cells. It is upregulated under inflammatory conditions as well as in skin lesions where endothelial cell proliferation occurs. Endoglin has the capacity to bind transforming growth factor beta (TGF-beta) and can reduce the bioavailability of TGF-beta. TGF-beta has a growth-inhibiting effect on keratinocytes and a restraining influence on the extravasation of peripheral white blood cells. In order to find out how endoglin is expressed in the margin zone of psoriatic plaques and how it correlates with the appearance of an inflammatory infiltrate, punch biopsies were taken from the margin zone of actively spreading psoriatic plaques in 8 patients. Indirect immunoperoxidase staining was performed using PAL-E (vascular endothelium), PN-E2 (anti-endoglin) and T11 (T-lymphocytes). In all patients it was found that the appearance of parakeratosis correlated with a clear increase of PN-E2 expression. PAL-E and PN-E2 expression was assessed, using a 5-point scale. Thus a tendency to decreased PN-E2 expression in uninvolved skin compared to PAL-E expression was found within the margin zone (1.6 +/- 0.4 and 2.2 +/- 0.4, respectively), whereas in involved skin PN-E2 expression and PAL-E expression were in agreement (2.6 +/- 0.5 and 2.6 +/- 0.5 respectively), suggesting that in the overt plaque all endothelium is in a so-called activated state. Also correlating with PN-E2 expression was the appearance of a huge dermal lymphocytic infiltrate and epidermal T-lymphocytic expression. The present study lends further support for a permissive role of endoglin expression in the development of the psoriatic lesion.