Antiestrogens target the estrogen receptor and counteract the growth stimulatory action of estrogen on human breast cancer. However, acquired resistance to antiestrogens is a major clinical problem in endocrine treatment of breast cancer patients. To mimic acquired resistance, we have used a model system with the antiestrogen sensitive human breast cancer cell line MCF-7 and several antiestrogen resistant cell lines derived from the parental MCF-7 cell line. This model system was used to study the expression and possible involvement in resistant cell growth of insulin-like growth factor binding protein 2 (IGFBP-2). By an oligonucleotide based microarray, we compared the expression of mRNAs encoding insulin-like growth factor binding protein 1,2,3,4,5 and 6 (IGFBP-1 to -6) in the parental MCF-7 cell line to three human breast cancer cell lines, resistant to the antiestrogen ICI 182,780 (Faslodex/Fulvestrant). Only IGFBP-2 mRNA was overexpressed in all three resistant cell lines. Thus, we compared the IGFBP-2 protein expression in MCF-7 cells to nine antiestrogen resistant breast cancer cell lines, resistant to either ICI 182,780 or tamoxifen or RU 58,668 and found that IGFBP-2 was overexpressed in all nine resistant cell lines. Three of the resistant cell lines, resistant to different antiestrogens, were selected for further studies and IGFBP-2 overexpression was demonstrated at the mRNA level as well as the intra- and extracellular protein level. The objective of this study was to examine if IGFBP-2 is involved in growth of antiestrogen resistant human breast cancer cells. Therefore, IGFBP-2 expression was inhibited by antisense oligonucletides and siRNA. Specific inhibition of IGFBP-2 protein expression was achieved in MCF-7 and the three selected antiestrogen resistant cell lines, but no effect on resistant cell growth was observed. Thus, we were able to establish IGFBP-2 as a marker for antiestrogen resistant breast cancer cell lines, although IGFBP-2 was not a major contributor to the resistant cell growth.