The use of platinum complexes for the therapy of breast cancer is an emerging new treatment modality. To gain insight into the mechanisms underlying cisplatin resistance in breast cancer, we used estrogen receptor-positive MCF-7 cells as a model system. We generated cisplatin resistant MCF-7 cells and determined the functional status of EGFR, MAPK, and AKT signalling pathways by phospho-receptor tyrosine kinase- and phospho-MAPK arrays. The cisplatin-resistant MCF-7 cells are characterized by increased EGFR phosphorylation, high levels of AKT1 kinase activity, and ERK1 phosphorylation. In contrast, the JNK and p38 MAPK modules of the MAPK signalling pathway were inactive. These conditions were associated with inactivation of the p53 pathway and increased BCL2 expression. We investigated the expression of genes encoding the ligands for the ERBB signalling cascade and found a selective upregulation of amphiregulin expression, which occurred at later stages of cisplatin resistance development. Amphiregulin is a specific ligand of the EGF-receptor (ERBB1) and a potent mitogen for epithelial cells. After exposure to cisplatin, the resistant MCF-7 cells secreted amphiregulin protein over extended periods of time and knock down of amphiregulin expression by specific siRNA resulted in a nearly complete reversion of the resistant phenotype. To demonstrate generality and importance of our findings, we examined amphiregulin expression and cisplatin resistance in a variety of human breast cancer cell lines and found a highly significant correlation. In contrast, amphiregulin levels did not significantly correlate with cisplatin resistance in a panel of lung cancer cell lines. We have, thus, identified a novel function of amphiregulin for cisplatin resistance in human breast cancer cells.