The authors studied the role of 70-Kd heat shock protein (HSP70) in the progression of breast cancer by examining the correlation between the expression of HSP70 and epidermal growth factor receptor, c-erbB-2, p53, and estrogen receptor in 124 cases of invasive primary human breast cancers. Positivity of an anti-HSP70 monoclonal antibody, C92, was closely associated with the elevation of estrogen receptor (P < .008), whereas it inversely correlated with the expression of p53 (P < .01). In addition, the expression of HSP70 correlated inversely with the expression of epidermal growth factor receptor, although the correlation was not statistically significant (P = .06). These results suggest that the expression of HSP70 plays a role in the progression of human breast cancer.