Metallothionein (MT) in tumor cells has been implicated as one of the factors involved in mechanisms of resistance to anti‐cancer drugs, including cis‐diaminedichroloplatinum (CDDP) and adriamycin (ADM). The relationship between the expression of MT and chemotherapy with anti‐cancer drugs was studied in CDDP‐ and ADM‐resistant human bladder cancer cell lines and tissue samples from clinical cases. In drug‐resistant cell lines (T‐24/ADM, CI‐7/CDDP) established in our laboratory, MT expression was studied by immunohistochemistry using the avidin‐biotin peroxidase complex (ABC) method and radioimmunoassay (RIA), using anti‐MT antibody. In addition, other potentfal mechanisms of drug resistance, such as P‐glycoprotein expression were examined and the levels of reduced glutathione (GSH), oxidized glutathione (GSSG) and glutathione‐S‐transferase (GST) determined in these cell lines. The results of these investigations demonstrate that the expression of MT in resistant cell lines increased 2.1‐ and 2.5‐fold when compared with parent cell lines (CI‐7, T‐24). GSH, GSSG and GST levels were unchanged and P‐glycoprotein was not over‐expressed. A total of 120 tissue samples from 35 clinical cases of bladder cancer, before and after chemotherapy, were stained for MT which was detected in 10 of the 35 cases before chemotherapy. The incidence of MT expression was significantly higher (p less than 0.05) in cases with lower pathological tumor grades. By analyzing the MT staining after chemotherapy in the cases whose MT staining was negative before chemotherapy, it was found that cases receiving continuous administration (intravesical chemotherapy or peroral chemotherapy) showed a higher incidence (9/13) of positive staining for MT, than patients receiving intermittent administration (intravenous chemotherapy) (1/8), (p less than 0.05). These results demonstrate that: 1) a correlation exists between MT expression and tumor differentiation and 2) repetitive and continuous administration of anti‐cancer drugs results in increased MT expression in bladder cancer cells. MT expression may therefore be one of the mechanisms by which urothelial tumors acquire resistance to anti‐cancer drugs. Copyright © 1994, Wiley Blackwell. All rights reserved