Metallothioneins (MT) are low molecular weight proteins present both in normal and in neoplastic cells. They represent the main mechanism of the cell which protects it from action of heavy metal ions and the principal zinc-binding proteins. MT act as controllers of zinc-dependent enzymes, participating, i.a., in cell proliferation processes. The latter is of basic significance also in neoplastic diseases. Despite the relatively short period of studies on the role of MT in neoplastic processes, several data confirm their prognostic significance. Such data have been obtained in studies on expression of the proteins in breast, renal, urinary bladder, ovarian, laryngeal and lung cancers. MT are thought to play certain role in carcinogenesis, as indicated by results of studies on malignant tumors of large bowel, liver and stomach. The frequently noted positive correlation between MT expression and the expression of Ki-67 and PCNA antigens points to role of the proteins in the cell proliferation mechanism. Results collected and discussed in present paper, obtained by various authors and related to the role of MT expression in cells of various malignant tumors suggest a potential for using the protein as a prognostic factor in neoplastic diseases.