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No / Objectives Previous studies have demonstrated that intravesical administration of apaziquone (EOquin) has ablative activity against superficial bladder cancer marker lesions with 8 out of 12 complete responses recorded. We present a comparison between the rates of tumor recurrence before and after treatment with apaziquone. Methods The rate of tumor recurrence after treatment with apaziquone was compared with each patient's historical record of recurrences obtained from a retrospective analysis of the patients' case notes. The time to each recurrence event before apaziquone treatment and the time to the first recurrence after apaziquone treatment were recorded, and the data were analyzed using a population-averaged linear regression model using Stata Release, version 9.2, software. Results Of the eight complete responses obtained in the Phase I study, tumor recurrence occurred in 4 patients and the remaining 4 patients remained disease free after a median follow-up of 31 months. The time to the first recurrence after apaziquone treatment was significantly longer (P <0.001) compared with the historical pattern and recurrence interval before apaziquone. Before apaziquone instillation, the mean ± SE recurrence rate and tumor rate per year was 1.5 ± 0.2 and 4.8 ± 1.2, respectively, and these decreased to 0.6 ± 0.25 and 1.5 ± 0.8, respectively, after apaziquone treatment (P <0.05). Conclusions The results of this study indicate that early recurrences after treatment with apaziquone are infrequent and the interval to recurrence is significantly greater compared with the historical recurrence times for these patients. Larger prospective randomised trials are warranted to confirm these results. Aapaziquone (EOquin, USAN, E09, 3-hydroxy-5-aziridinyl-1-methyl-2[indole-4,7-dione]¿prop-¿-en-¿-ol) belongs to a class of anticancer agents known as bioreductive drugs that require metabolism by cellular reductases to generate a cytotoxic species.1 Although it is chemically related to mitomycin C, apaziquone has a distinctly different mechanism of action and preclinical activity profile.1 and 2 The initial optimism generated by its preclinical activity profile rapidly evaporated after the demonstration that intravenously administered apaziquone was clinically inactive against a range of solid tumors in Phase II clinical trials.3 and 4 Several possible explanations were considered for its lack of efficacy, but poor drug delivery to the tumor because of the rapid pharmacokinetic elimination of apaziquone in conjunction with relatively poor penetration through avascular tissue was considered to be the principal reason.5 On the basis of the rationale that intravesical administration would circumvent the problem of drug delivery and any apaziquone absorbed into the blood stream would be rapidly cleared,6 a Phase I-II clinical pilot study of intravesical administration of apaziquone to superficial bladder tumors was established.7 The results of that trial demonstrated that intravesically administered apaziquone has ablative activity against superficial bladder transitional cell carcinoma (TCC) marker lesions.7 These results were confirmed and extended in a Phase II clinical trial of 47 patients with superficial bladder TCC, in which complete responses were obtained in 67% of patients.8 Because all the enrolled patients in the original trial7 had had multiple recurrences after previous intravesical chemotherapy and/or immunotherapy, the purpose of the present study was, first, to report the recurrences that occurred after apaziquone treatment and, second, to study the effect of apaziquone instillation on the recurrence rate by statistically comparing these results with the historical pattern of recurrences for each patient before treatment with apaziquone.