Heat shock protein Mr 27,000 (hsp27) is found in many human breast cancer cells and tissues; its expression is associated with the presence of estrogen receptors, lower cell proliferation, and resistance to certain chemotherapies. The purpose of this study was to assess whether hsp27 may be present in sera from women with primary breast cancer and to know whether autoantibodies to hsp27 may be found in these patients. The study was performed by Western blot analyzing sera from 42 normal premenopausal women, 20 normal postmenopausal women, and 36 breast cancer patients. hsp27 was clearly detected in sera by immunoblotting but only after immunoprecipitation. The mean hsp27 levels in cancer patients were higher than in the control patients; however, 66% of the breast cancer patients showed hsp27 within the normal range, indicating low sensitivity. Moreover, cancer patients with metastatic disease did not show significantly higher hsp27 levels than cancer patients without metastases. Serum hsp27 levels did not correlate with the hsp27 levels in tumor tissues detected by immunohistochemistry. Elevated CA 15-3 levels were not associated with high hsp27 values. Autoantibodies against hsp27 were not detected by immunoblotting in normal sera and in sera from breast cancer patients. As a consequence, serological determination of this biomarker is unlikely to be of utility in the detection and follow-up of breast cancer patients.