Expression of the pS2 protein in breast carcinoma is a useful guide to prognosis and response to tamoxifen. We have investigated pS2 protein expression in both the primary tumour and lymph node metastases (LNM) using a computer-assisted image analysis system. In a consecutive series of 208 patients undergoing surgical excision of primary breast cancer with axillary clearance, 89 patients were found to have involved lymph nodes. We found a highly significant correlation between pS2 expression in primary tumours and their LNM when 5% was taken as the cut-off for positive staining (Fischer Exact, P < 0.0001). There was also a highly significant correlation between the proportion of positive staining between the local metastases and primary tumours (Spearman's rank order correlation = 0.87; P < 0.0001). We conclude that the pS2 status of LNM can be accurately predicted from the pS2 status of the primary tumour. As such, appropriate adjuvant therapy for primary breast cancer, or second line therapy for disseminated disease can be selected on the pS2 status of the primary tumour alone.