Background: Voided urine cytology has been used as an adjunct in the diagnosis of non-muscle invasive bladder cancer (NMIBC), with a sensitivity and specificity ranging between 13-75% and 76-100% respectively. There is limited data on the accuracy and utility of cytology following BCG therapy. We reviewed the results of cytology in patients undergoing induction and maintenance BCG immunotherapy in our institution. Methods: Newly diagnosed patients who had received induction and maintenance intravesical BCG therapy from 2004 - 2019 were identified from a prospective database and their outcomes reviewed retrospectively. Histopathology results of biopsies / resected specimens and voided urine cytology results were examined for 273 patients. Results: A total of 2567 cytology results and 638 biopsy results were recorded. The average age was 73.2 years and median number of BCG treatments was four (induction followed by three maintenance courses). Median follow up was 38 months. 94 patients (34.4%) had recurrence following BCG therapy. Of those 33 patients (12.1%) had progression to muscle invasive disease. The number of cytology samples per patient after BCG therapy ranged from 1-23 (median 7), with several patients having repeated, potentially unnecessary negative urine cytology. Overall accuracy of cytology (n = 526) was sensitivity 44.2%, specificity 84.7%, PPV 38.9%, NPV 87.3%. Patients that had an erythematous bladder or red patch at flexible cystoscopy underwent subgroup analysis; this gave a very high NPV of 95.9%, with additional sensitivity being 65.5%, specificity 85.9% and PPV 33.3%. Number of positive cytology results (Chi² = 44.30, P = 0.002), any positive cytology (Chi² = 27.94, P < 0.001) and positive cytology after induction BCG therapy (Chi² = 30.381, P < 0.001) were all strongly associated with recurrence. Conclusions: Positive urine cytology in patients undergoing intravesical BCG therapy predicts increased risk of recurrence and has good specificity. We would recommend using voided urine cytology in patients who have an erythematous bladder or red patch at flexible cystoscopy. If the cytology is positive then proceed to biopsy, however, if it is negative continue with surveillance.