Surgical specimens of the normal kidney and of renal cell carcinoma (RCC) tissues at different stages of progression and of various histological grades were examined for the expression of MUC1 mucins with sialylated carbohydrates (sialylated MUC1 mucins) using a monoclonal antibody MY.1E12. Immunohistochemical studies revealed that the binding sites for this antibody were localized to the apical side of the epithelial cells of the distal convoluted tubules, Henle's loops and collecting ducts. However, proximal convoluted tubules, where RCC is considered to originate, were not stained. This antibody also bound strongly to RCC at advanced stages of progression and at metastatic sites, and to RCC of histologically high grades (undifferentiated). The epitope, presumably sialylated MUC1 mucin, was detected not only along the surface of the cell membranes but also in the cytoplasm. The level of expression of sialylated MUC1 mucins was inversely correlated with the survival of the patients with RCC and the disease-free survival period after curative surgery. Western blot analysis demonstrated that the electrophoretic mobility of sialylated MUC1 mucins of RCC was greater than that from the normal kidney. It is suggested that high levels of expression of sialylated MUC1 mucins in certain human RCC populations correlate with the aggressiveness of the disease, such as the tendency to form metastasis.