Breast cancer (BC) is the most frequent cause of cancer death in women throughout the world. Thus, it is necessary to establish sensitive screening, diagnosis and treatment methods for BC. Heat shock protein 70 (HSP70) is an important cellular stress response protein that protects cells from apoptosis. Recent studies have shown that serum HSP70 levels may provide clinically important information in various types of cancer. HSP70 is also overexpressed in BC, which is known to be associated with cancer progression, apoptosis and cell proliferation. However, the serum level of HSP70 and its diagnostic and prognostic potential in BC have not been investigated yet. The aim of this study was to determine the usefulness of serum HSP70 level as a diagnostic test and its predictive value in patients with BC. This prospective study consisted of 45 female patients diagnosed with BC and 16 healthy women who were matched for age and body mass index (BMI). Enzyme-linked immunosorbent assay (ELISA) technique was used to measure the serum level of HSP70. The serum level of HSP70 was significantly higher in patients with BC than in the healthy control group (5.98 +/- 2.05 vs. 1.49 +/- 0.47 ng/ml, p = 0.001). HSP70 level > 2.41 ng/ml was the best cutoff value to predict BC (97.78% sensitivity and 93.75% specificity). This study shows that HSP70 can be used as an adjunct to other diagnostic tests for BC and may be helpful for identifying patients at increased risk of BC.