This study was undertaken to determine the expression of p53 gene in cytologic specimens from benign and malignant breast lesions. To detect p53 an immunocytochemical assay with p53 (pAb421) monoclonal antibody was used. Abnormalities in p53 expression were found in 19 out of 40 Fine Needle Aspiration (FNA) smears with infiltrating ductal breast carcinomas. Benign epithelial breast cells obtained from fibroadenomas, fibrocystic disease and smears from nipple discharge reacted negatively for p53 in 38 out of 39 cases. Moderate positive reaction, confined to a few clusters of epithelial cells, was observed in one smear of fibroadenoma with cellularity. The results recorded in this study show that no significant association was found between p53 staining and stage of disease, tumor size or nodal status and that the immunocytochemical assay represents a simple method for the detection of p53 associated proteins in breast lesions.