BACKGROUND: It has been shown that CD44 variants are differentially expressed in normal and neoplastic breast tissues. The diagnostic value of these markers in distinguishing benign from malignant breast lesions has not been well examined. AIMS: To evaluate the diagnostic value of CD44s in distinguishing between intraductal papillomas and papillary carcinomas of the breast, which may be difficult morphologically. METHODS: Expression of CD44s detected by immunohistochemistry was studied in a series of intraductal papillomas (11) and papillary carcinomas (10). The normal breast tissues surrounding the lesions of these cases served as a control. The number of CD44s positive epithelial cells was scored and categorised as < 10%, 10-70%, or > 70%. RESULTS: Normal breast epithelial cells and all intraductal papillomas (11 of 11) expressed CD44s in a high proportion of cells (> 70%). In contrast, the majority of papillary carcinoma cases (eight of 10) expressed this marker in < 10% of the cells. In the remaining two papillary carcinoma cases, positivity was seen in more than 10% but still less than 70% of the cells. CONCLUSIONS: CD44s detection by immunohistochemistry is useful in distinguishing intraductal papillomas from papillary carcinomas of the breast.