Nitric oxide (NO) is generated by a family of enzymes, nitric oxide synthases (NOS), in a wide range of mammalian cells. NO produced by the inducible NOS isoform (iNOS) has been suggested to play an important role in tumor biology with both tumor promoter and antitumor activity. Here, the cellular localization of iNOS in tissue of 100 cases of urinary bladder cancer was assessed immunohistologically using a commercially available antiserum. Positive iNOS immunostaining was detected in all samples of tumor tissue, whereas nonmalignant tissue adjacent to malignant areas did not show any iNOS positivity. The tumor tissue revealed a highly inhomogeneous staining pattern. In addition to uniformly stained tumor specimens, we also found markedly iNOS-positive tumor islets in the midst of unstained tumor tissue and scattered individual tumor cells expressing marked staining. In some cases, the tumor tissue showed no or only weak staining intensity. In some instances, the superficial epithelial layer of papillary carcinomas was extremely immunoreactive, in other cases it was not. Thus we were unable to show a clear correlation to tumor grade or stage. Further studies with a diversity of tumor markers including molecular genetics techniques will be necessary to elucidate how and to what extent NO and bladder cancer of different grades and stages are functionally interrelated.