The critical mechanisms responsible for antiestrogen resistance have not yet been elucidated. We previously established a breast cancer cell line, KPL-1, derived from a patient with recurrent disease which appeared under tamoxifenadministration. In a previous study, we suggested that this cell line is estrogen receptor (ER)-positive but tamoxifen-resistant. In the present study, the effects of a pure antiestrogen, ICI 182,780, on this cell line were investigated. Although tamoxifen inhibited neither cell growth nor estradiol-stimulated transcriptional activity in vitro, ICI 182,780, significantly inhibited both of them. Tamoxifen and ICI 182,780 were then administered to female nude mice bearing KPL-1 tumors. Tamoxifen had no effect on tumor growth, but ICI 182,780 unexpectedly stimulated it (p = 0.022). Estradiol tended to inhibit tumor growth (p = 0.198). Immunohistochemical analysis revealed that ICI 182,780 significantly increased the Ki6-labeling index (p<0.001) but estradiol decreased it (p = 0.035). To explore the possible mechanisms of these phenotypes, the mRNA levels of ER-alpha,ER-beta, transforming growth factor-beta1, fibroblast growth factor (FGF)-1 and FGF-4 in KPL-1 cells were compared with those in other ER-positive human breast cancer cell lines by reverse-transcription polymerase chain reaction. FGF-1 was overexpressed only in KPL-1 cells. This cell line is the first breast cancer cell line to be growth-stimulated by ICI 182,780 in vivo. Paracrine interaction between tumor cells and stromal cells mediated by growth factors, such as FGF-1, might be a key factor to explain the unique hormone responsiveness of KPL-1 cells.