Objective. To determine the incidence of positive urinary nuclear matrix protein 22 (NMP22) values, which are currently used to detect transitional cell carcinoma of the bladder, in renal cell carcinoma (RCC). Material and methods. Urinary NMP22 values were determined preoperatively in 41 patients in whom a solid renal mass had been detected using CT and who were scheduled for radical nephrectomy; 38 of these patients were diagnosed with RCC. Two patients had xanthogranulomatous pyelonephritis and one had metastasis of a small cell adenocarcinoma to the kidney; these patients were excluded from the study. A total of 30 patients with kidney stones and simple renal cysts were used as controls. Results. The urinary NMP22 values of the RCC patients were significantly higher than those of the controls. Of the 38 patients with RCC, 23 (60.5%) had positive urinary NMP22 values >= 10 U/ml. There were four measurements above this cut-off level in the control group. Urinary NMP22 values increased with an increase in pathologic tumor stage, but the correlation was not statistically significant. There was no correlation between grade and urinary NMP22 or between tumor burden and urinary NMP22. Conclusions. The urinary NMP22 test may help to diagnose RCC and may also result in an increase in the incidental discovery of RCC. As elevated urinary NMP22 levels have also been found to occur in RCC, patients with suspected bladder cancer and positive urinary NMP22 levels should be more broadly evaluated. Specific NMP assays for renal tumor cells may increase the utility of the test for RCC.