To investigate the utility of fluorescence in situ hybridization (FISH) in secondary electroresection of bladder cancer.From January 2016 to April 2022, bladder cancer patients who had undergone secondary electroresection in Tongji Hospital and had preoperative urine FISH were recruited, and the positive rate, accuracy, sensitivity, specificity, genetic material changes and predictive power on malignancy degree of FISH in the secondary electroresection of bladder cancer were examined.Twenty-six patients with bladder cancer were included in this study, and 8 were confirmed by secondary electroresection, including 6 cases positive for FISH positive and 2 negative for FISH. Besides, among the subjects, 18 were without tumor recurrence, including 1 case with positive FISH results and 17 with negative FISH results. Tumor recurrence was diagnosed in 85.71% (6/7) of FISH-positive patients in secondary electroresection while only 10.53% (2/19) of FISH-negative patients were found to develop tumor recurrence in the secondary electroresection. The sensitivity of FISH for the detection of bladder cancer before secondary electroresection was 75%, with a specificity of 94.44%, and an accuracy of 88.46%. A 6-month follow-up revealed that 2 of the 8 recurrent patients underwent radical resection of bladder cancer, and the remaining 6 patients had no recurrence, as confirmed by regular bladder perfusion and microscopy. In the 18 non-recurrent patients during secondary electroresection, no recurrence developed.Urine FISH can achieve a high detection rate and specificity for secondary electroresection of bladder cancer. If a bladder cancer patient who are indicated for secondary electroresection is negative for urine FISH, the recurrence rate after secondary electroresection will be low, and the cystoscopy can be performed before deciding whether to perform secondary electroresection.© 2013-2023 Bladder, All rights reserved.