Intravesical oxybutynin chloride has been reported to be effective for overactive bladder, although sometimes the efficacy does not last long enough. To improve this deficiency, we report the effects of intravesical oxybutynin chloride with hydroxypropylcellulose (modified intravesical oxybutynin). Modified intravesical oxybutynin (5 mg/10 mL, twice a day) was administered to six overactive bladder patients for more than 1 year (two men and four women; average age, 56.5 years) who did not respond to oral anticholinergic agents and electric stimulation. Cystometography (CMG) was performed before, 2 hours, and 1 week after the start of modified intravesical oxybutynin. In addition, plasma levels of oxybutynin and its active metabolite, N-desethyl-oxybutynin (DEOB), were measured by high-performance liquid chromatography before, 1, 2, and 4 hours after the initial treatment of modified intravesical oxybutynin. CMG studies revealed that two of the six patients did not demonstrate uninhibited contractions 1 week after the treatment and that cystocapacity of before, 2 hours, and 1 week after the initial modified intravesical oxybutynin was 141.8+/-15.3, 210.0+/-35.5, and 305.0+/-21.3 mL, respectively. Plasma levels of oxybutynin and DEOB before, 1, 2, and 4 hours after the first instillation of modified intravesical oxybutynin were oxybutynin; not detected, 8.8+/-2.5, 6.8+/-1.1, 3.0+/- 1.0 ng/ml, and DEOB; not detected, 4.2+/-1.3, 6.4+/-1.7, 5.1+/- 1.4 ng/ml, respectively. No side effects were observed in any of the patients. Modified intravesical oxybutynin is an effective and safe therapy option for overactive bladder patients who do not respond to other treatments such as oral anticholinergic agents and electric stimulation.