pS2 expression was studied in 212 breast carcinomas. Immunocytochemical assays (ICAs) were performed on frozen sections and evaluated by digital image analysis. pS2 immunostaining was compared in frozen sections and paraffin sections. The pS2 positivity was observed in 45% of the cases on frozen sections. But in pS2 positive tumors, the tumor surface which was immunostained was small (m=14.3%). In 22% of immunoreactive tumors the positive surface was 5% or less. In contrast to frozen sections, the pS2 positivity on paraffin sections could not be evaluated by image analysis system because of the background. No significant correlation was observed between pS2 expression and patient age, tumor size, histological type and grade, nor with lymph node status. The pS2 positivity was significantly correlated to ER and PR positive immunodetection on frozen sections. But pS2 was independent from pHER-2/neu and cathepsin expression whereas there was a significant inverse relationship between pS2 and Ki67. This study shows that pS2-ICAs on frozen sections and image analysis are optimal and standardized conditions for the evaluation of pS2 expression in breast carcinomas, and suitable for selecting patients for hormone therapy.