A prospective randomized study was conducted to determine the ablation capacity of mitoxantrone in Ta-T1 superficial bladder tumors versus mitomycin C, a drug whose intravesical ablation properties are well-known. Fifty-seven patients comprised the study. The tumor was not completely resected when the patient underwent TUR. This residual tumor was used as control. The patients were treated with either 20 mg Mitoxantrone or 40 mg mitomycin C weekly for 8 weeks and two other instillations every 15 days in 50 ml saline solution. Response to therapy was evaluated between the 4th and 8th week and classified as complete response (CR), defined as no gross and microscopic evidence of residual tumor, or no response (NR) or therapeutic failure. CR was observed in 77.7% of the patients treated with mitomycin C and in 50% in those that had been treated with Mitoxantrone. Treatment was discontinued because of side effects in 15% of the patients treated with mitomycin C and in 63.4% of those who received Mitoxantrone. We can conclude from the results of the present study that Mitoxantrone is a useful agent for ablation therapy of superficial bladder tumors, although the high incidence of severe side effects warrants its limited use and at high dilutions.