p53 protein has been frequently detected at high levels in the nuclei of human breast cancer cells. We analyzed immunohistochemically the association between nuclear localization of p53 protein and clinical and histological parameters of breast cancer patients. Surgically resected tissues of 73 primary breast cancers were processed by acetone fixation and paraffin embedding and examined using an anti-p53 monoclonal antibody, PAb1801. p53 immunoreactivity was detected in the nuclei of cancer cells in 17 cases (23%). The nuclear p53 immunoreaction was closely associated with overexpression of c-erbB-2 protein (P less than 0.05), high histologic grade (P less than 0.01), advanced clinical stage (P less than 0.05), and negative estrogen receptor status (P less than 0.01). When 31 cases which had been followed up for more than 50 months were examined, a positive nuclear p53 immunoreaction was found to be significantly associated with shorter overall survival of patients (P less than 0.01). These results suggest that immunohistochemical examination of nuclear p53 protein is clinically useful as an indicator of breast cancer aggressiveness.