['We explored the clinical usefulness of serum carbonic anhydrase 9 as a potential biomarker for conventional renal cell cancer.', 'This study included 91 patients with conventional renal cell cancer and 32 healthy individuals. Enzyme linked immunosorbent assay was used to measure the carbonic anhydrase 9 level. A followup (median 38 months) was performed to track early recurrence after surgery for patients with localized disease. Recurrence-free survival curves were calculated by the Kaplan-Meier method and compared using the log rank test.', 'The mean serum carbonic anhydrase 9 level in patients with metastatic conventional renal cell cancer (216.68 +/- 67.02 pg/ml) or localized conventional renal cell cancer (91.65 +/- 13.29 pg/ml) was significantly higher than in healthy individuals (14.59 +/- 6.22 pg/ml, p <0.001 and p = 0.001, respectively). The mean serum carbonic anhydrase 9 level in patients with metastatic conventional renal cell cancer was significantly higher than in those with localized disease (p = 0.004). Of patients with localized disease those with recurrence had a significantly higher serum carbonic anhydrase 9 than those without recurrence (p = 0.001). On univariate analysis serum carbonic anhydrase 9, tumor stage, tumor grade and tumor size were associated with recurrence. The recurrence-free survival curve indicates that patients with a high serum carbonic anhydrase 9 level had a significantly higher recurrence rate than those with a low serum carbonic anhydrase 9 (p = 0.001).'] Is serum carbonic anhydrase 9 level associated with postoperative recurrence of conventional renal cell cancer?