The cells of all organisms respond to environmental stress by synthesizing a specific group of proteins called heat shock proteins (HSPs). While the protective role of HSPs has been observed against cellular stress, recent reports have suggested a number of functions of HSPs in various cellular processes in normal and pathological conditions. HSP27 is an important low molecular weight HSP (Mr: 27,000) found in human cells. HSP27 appears to be involved in intracellular signaling and drug resistance in addition to thermotolerance. In human breast tumors, a striking association between HSP27 overexpression and a shorter disease-free survival period was reported. In this paper, we evaluated the expression levels of HSP27 in the cell lines of human breast and other tumors. Western blotting of HSP27 has shown the correlation of HSP27 expression and metastasis and tumorigenicity of mammary and prostate tumors. Thus the biological significance of HSP27 in aggressive mammary and prostate tumors was indicated. HSP27 is suggested to be a prognostic marker for the malignancy of mammary and prostate tumors.