Objective: To investigate whether the timing of an immediate instillation of mitomycin C (on the day of transurethral resection of bladder tumour [TURBT] or 1 day later) has an impact on time to recurrence of non-muscle-invasive bladder cancer (NMIBC). Patients and Methods: All patients with NMIBC who were enrolled in a prospective trial between 1998 and 2003, and treated with an early mitomycin C instillation (on the day of TURBT or 1 day later), were selected. Statistical analysis was performed with Kaplan-Meier curves and multivariable Cox regression. Results: Administering an instillation of mitomycin C on the day of TURBT or 1 day later did not show a statistically significant difference in time to recurrence in a univariable model (log-rank P = 0.99). After correcting for the number of scheduled adjuvant instillations, no statistically significant difference could be detected either: hazard ratio 1.05 (95% confidence interval 0.81-1.35, P = 0.74). Conclusion: These data do not support the hypothesis that a very early instillation (on the day of TURBT) of mitomycin C decreases the risk of recurrence as compared with an early instillation (1 day after TURBT).