This study is designed to assess the efficacy and safety of DTA-H19/PEI given as six intravesical instillations of 20 mg of plasmid DNA complexed with PEI into the bladder of patients with intermediate risk superficial bladder cancer [recurrent stages Ta (low or high grade)and T1, (low grade) transitional cell carcinoma (TCC)] who have failed prior intravesical therapies including either Bacillus Calmette-Guérin (BCG) or chemotherapy. The primary efficacy objective is to determine the effect of DTA-H19/PEI on the prevention of new tumors after the induction course of 6 weekly intravesical administrations of investigational product assessed 8 to 10 weeks after the start of treatment. Secondary objectives include assessing the ablative effect of DTA-H19/PEI on a marker tumor, safety assessed by the incidence and severity of adverse events, determining the long-term (46 weeks) continued rates of absence of bladder cancer, and time to tumor recurrence in those patients who had a complete response (CR) after the induction course.