Insulin-like growth factor (IGF) system is one of the growth factor systems that are believed to modulate steroid hormone actions in the endometrium through autocrine/paracrine mechanisms. IGF-I and IGF-II stimulate proliferation and differentiation, and maintain differentiated cell functions in several cell types in vitro. Endometrial stromal cells produce IGF-I and IGF-II as well as the high affinity IGF-binding proteins (IGFBP), whereas epithelial cells and, in a lesser amount, stromal cells contain cell membrane receptors for IGF. Oestrogen stimulates IGF-I gene expression, and IGF-II gene expression is associated with endometrial differentiation. The mRNA of six high affinity IGFBPs, which can modulate IGF actions, are expressed in human endometrium. The most abundant IGFBP in human endometrium is IGFBP-1, which is secreted by predecidualized/decidualized endometrial stromal cells in late secretory phase and during pregnancy. The primary negative regulator of IGFBP-1 production is insulin. IGFBP-1 competes with type I IGF receptor for binding of IGF in the endometrium and in cultured human trophoblastic cells. IGF-I mRNA is suppressed and mRNA encoding IGF-II and IGFBP-1 are consistently up-regulated in decidualized endometrium in women treated with the intrauterine levonorgestrel system (LNG-IUS). Strong cytoplasmic staining for IGFBP-1 was detected in decidualized endometrium in women using LNG-IUS for contraception or for endometrial protection during post-menopausal oestrogen replacement therapy. Simultaneously, oestrogen receptors were present, while progesterone receptors were hardly detectable in the endometrium by immunohistochemistry. The latter findings suggest that suppression of IGF-I action by IGFBP-1 may be one of the molecular mechanisms accounting for progestagenic and anti-oestrogenic effects of LNG-IUS in the endometrium. Consequently, examination of local IGF-I, IGF-II and IGFBP-1 expression might provide additional information when evaluating the effect of different progestins on the endometrium at the molecular level.