The goal of the present study was to determine if the expression of metallothionein isoform 3 (MT-3) might serve as a biomarker for human bladder cancer. To accomplish this goal, we defined the localization and expression of MT-3 protein and mRNA using fresh and archival biopsy specimens obtained from patients undergoing differential diagnosis for a variety of bladder disorders. We used immunohistochemistry, immunoblot, and RT-PCR analysis to define the localization and expression of MT-3 protein and mRNA. Immunohistochemical analysis disclosed no immunoreactivity for MT-3 in normal bladder cells. The absence of MT-3 expression in the normal bladder was further confirmed by demonstrating that MT-3 mRNA could not be detected using reverse transcriptase-polymerase chain reaction (RT-PCR) or MT-3 protein using immunoblot. Immunohistochemistry also disclosed no immunoreactivity for MT-3 in archival biopsy specimens from patients with interstitial cystitis and related disorders. Immunohistochemical analysis demonstrated that MT-3 was expressed in carcinoma in situ (CIS), high-grade bladder cancer, low-grade bladder cancer, and dysplastic lesions. MT-3 immunostaining was intense in both CIS and high-grade bladder cancer, and low to moderate in low-grade bladder cancer and dysplastic lesions. We determined MT-3 mRNA expression in a subset of these bladder cancer specimens; expression was elevated as compared to that of the housekeeping gene, ss-actin. The cDNA from the RT-PCR reaction primed for MT-3 contained a FokI restriction site, a site unique for MT-3 as compared to other MT family members. In conclusion, this study demonstrates that MT-3 is up-regulated in human bladder cancer and that this up-regulation increases with increasing tumor grade. The finding that MT-3 expression is minimal in normal bladder suggests that MT-3 might be developed into an effective biomarker for bladder cancer.