A key mechanism underlying the cyclical growth of the endometrium is its ability to regenerate a vascular capillary network. In normal cycling human endometrium, angiogenesis is influenced by both endocrine and paracrine factors. Hormonal manipulation of the endometrium, such as that occurring during the use of steroidal contraception, appears to result in capillary proliferation and fragility. As a consequence of these vascular changes, contraceptive users may be predisposed to unpredictable uterine bleeding, which is responsible for the high frequency of contraceptive discontinuation. In this paper we address mechanisms responsible for vascular endothelial cell proliferation in normal and contraceptive steroid-exposed endometria. We propose that regulation of endometrial angiogenesis is mediated indirectly, via steroid and cytokine actions on vascular endothelial growth factor (VEGF), and we present data indicating that VEGF expression in normal endometrial stromal cells is increased by oestrogens and progestins. Three proinflammatory cytokines with angiogenic effects in other systems (i.e. interleukin-1beta, tumour necrosis factor-alpha and interferon-gamma) do not appear to up-regulate VEGF expression in normal endometrial stromal cells. Well-characterized in-vitro models in conjunction with immunohistochemistry provide useful experimental systems to study endometrial neovascularization under physiological conditions and in those potentially perturbed via the use of contraceptive steroids.