Metallothionein (MT) plays a role in fundamental cellular processes such as proliferation, apoptosis and differentiation. We examined MT expression in women with invasive breast ductal carcinoma who underwent mastectomy/lumpectomy without neo-adjuvant treatment. We showed that MT was over-expressed in 87.9% of breast cancer tissues examined, with the mean percentage of positive cells at 30%. There were two patterns of MT expression: predominantly cytoplasmic in 75.9% and nuclear in 24.1% of MT-positive cases. Higher MT scores were associated with poorer histological grade (p = 0.009) but were independent of age, tumour size and oestrogen receptor status. For patients who were treated with adjuvant chemotherapy (cyclophosphamide/methotrexate/5 fluorouracil- or doxorubicin-based regimes), those with high MT expression had a significantly lower recurrence-free survival (p = 0.048), suggesting a role of MT in predicting disease recurrence. Down-regulation of MT in MCF-7 cells by silencing the MT-2A gene (the most abundantly expressed of the 10 known functional MT isoforms) increased chemosensitivity of the cells to doxorubicin. To examine the mechanisms underlying these clinical data, we used siRNAs to decrease MT-2A mRNA expression and protein expression. In MT down-regulated cells challenged with the IC(50) concentration of doxorubicin, we observed a significant reduction in cell viability. Cell cycle analysis also revealed a corresponding increase in apoptosis in the MT down-regulated cells following doxorubicin exposure, showing that down-regulation of MT increased susceptibility to doxorubicin cytotoxicity. The data suggest that MT could be a potential marker of chemoresistance and a molecular therapeutic target.