Abstract Introduction To evaluate the applicability of a modified Clavien classification system (CCS) in grading postoperative complications of transurethral resection of bladder tumours (TURB). Materials and methods A series of patients undergoing monopolar TURB from April 2011 to March 2012 at five Italian centers were enrolled. All complications occurring within the first 30-day postoperative period were prospectively recorded and graded according to the CCS. Results Overall, 275 patients were included. Median age was 71 (63/78) years; median BMI was 28 (25.4/30.8) Kg/m2 , median tumour size was 2 (1–3) cm; median number of tumour lesions was 1 (1–3). Median operative time was 30 (20/45) min. Fifty-seven complications were recorded in 43 patients. Overall postoperative morbidity rate was 16%. Most of the complications were not serious and classified as Clavien type I (42 cases; 74%) or II (8 cases, 14%). Higher grade complications were scarce: CCS IIIa in 1 case (2%) and CCS IIIb in six cases (10%). No TURB related death was reported. Six patients were re-operated due to significant bleeding or clot retention on postoperative days 2–7. On univariate (73.5 ± 38 versus 36.7 ± 21.6 min) and multivariate analysis longer operative time was an independent predictor of complications (OR: 1.06 per min, 95%CI 1.04–1.08, p  = 0.001). Conclusions A modified CCS can be used as a standardized tool to objectively define the complications of TURB which confirms to be a safe procedure with a low surgical morbidity. This tool can be used to aid in patient counselling and to facilitate scientific assessment.