A series of 80 female patients undergoing surgery for primary breast ductal infiltrating carcinoma not otherwise specified (NOS) was immunohistochemically studied in order to verify any relationships between Proliferating Cell Nuclear Antigen (PCNA) immunostaining, Heat Shock Protein 70 (HSP70) immunoreactivity, and several clinicopathological predictors. Positive PCNA scores (> 20% of strongly immunopositive malignant nuclei) were observed in neoplastic cells' nuclei in 13 tumors (16.25%) and were intimately associated with axillary nodal involvement (p = 0.0131), relatively high tumor grades (p = 0.0016), increased tumor size (p = 0.0312), and low or negative levels of estrogen receptors (p = 0.0323). HSP70 positive immunoexpression in malignant cells' cytoplasm (percentage of HSP70 immunoreactive cells > 10%) was detected in 33 samples (41.25%). It correlated significantly with presence of axillary lymph nodal metastases (p = 0.0033) and rather poor tumor differentiation (p = 0.0014), whereas an association of borderline statistical significance emerged between HSP70 immunoreactivity and high progesterone receptor status (p = 0.0637). PCNA positive immunostaining demonstrates the tumors' proliferative fraction and might be used as an indicator of increased malignant potential in breast cancer since it was associated with four adverse prognosticators. HSP70 immunodetection is a probable marker of the biological stress experienced by breast cancer cells, since it was related to relatively high tumor grades. Since both proteins may potentially predict disease outcome, their prognostic significance must be validated by direct relation to survival. A multivariate statistical analysis including the variables with which both proteins were associated will reveal any possible independent prognostic value of PCNA and HSP70 immunostaining in local, ductal invasive breast cancer NOS.