Abstract The immunohistochemical detection of tumor marker CA 50 was studied in bladder cancer of WHO grades I–III. The material consisted of tumors in 83 patients and the mean clinical follow-up time was thirteen years (range 9.6–22 years). The fraction of CA 50-positive cells (FPtot) in microscopic image was scored 0–100 percent. Also the maximally staining region was selected, and the fraction of CA 50-positive cells in this region was scored 0–100 percent (FPmax). The average staining intensity of CA 50-positive cells was scored from 0 to 3 in the whole section (ASItot) and in the maximally staining area (ASImax). The inverse relation between histologic grade, FPtot (p = 0.0001), and ASItot (p = 0.006) was statistically significant. FPtot (p = 0. 039) and ASItot (p = 0.018) were also inversely related to clinical stage. Occurrence of metastasis during the follow-up was associated with low CA 50 positivity (FPtot, p = 0.003; ASItot, p = 0.002). The lower the staining intensity or the lower the fraction of CA 50-positive cells, the more aggressive was the tumor. In survival analysis, low FPtot (p = 0.002) and ASItot (p = 0.007) values were related to high risk of bladder cancer death. The results show that immunohistochemical staining of bladder tumor specimens with CA 50 can be used to predict bladder cancer aggressiveness and survival.