Metallothioneins (MTs) are a set of low molecular weight proteins with a high binding affinity to metal ions. MT over-expression has been recently demonstrated in invasive ductal carcinoma of the breast with poor clinical prognosis. In the present study, MTs have been immunohistochemically investigated in normal human breast tissue and a variety of benign, pre-invasive, and malignant breast lesions. In normal breast tissue, MTs were present in myoepithelial cells whereas the vast majority of luminal cells were MT negative. In lesions without increased cancer risk (adenosis and scleradenosis), MT was only immunolocalized in myoepithelial cells. In papillomas, MT was also found exclusively in myoepithelial cells. In most cases of epitheliosis, both the luminal and myoepithelial cells expressed MT. Atypical lobular hyperplasia, lobular carcinoma in situ, and 13/15 invasive lobular carcinomas showed no MT over-expression. The two invasive lobular carcinomas with MT over-expression were classified as pleomorphic lobular carcinomas with apocrine differentiation. In contrast to lobular cancerization, 12/24 ductal in situ carcinomas and 9/20 invasive ductal carcinomas showed MT over-expression. In situ components found within invasive ductal carcinomas usually reflected the MT status of their invasive counterpart. It is concluded from our immunohistochemical results that breast carcinoma cases with MT overexpression arise from lesions which also show MT overexpression. Thus MT expression in carcinomas may be regarded as a genuine feature of the tumour cells and seems not to be related to endogenous or exogenous factors known to induce MT synthesis.