Purpose: We compared 200 U intradetrusor botulinum toxin A vs placebo in women with refractory idiopathic urge incontinence. Materials and Methods: This institutional review board approved, multicenter registered trial randomized women with refractory urge incontinence, detrusor overactivity incontinence and 6 or greater urge incontinence episodes in 3 days to botulinum toxin A or placebo at a 2:1 ratio. Refractory was defined as inadequate symptom control after 2 or more attempts at pharmacotherapy and 1 or more other first line therapies for detrusor overactivity incontinence. The primary outcome measure was time to failure, as evidenced by a Patient Global Impression of Improvement score of 4 or greater at least 2 months after injection, or changes in treatment (initiation or increase) at any time after injection. Safety data, including increased post-void residual volume, defined as more than 200 ml irrespective of symptoms, was obtained at specified time points. Results: Approximately 60% of the women who received botulinum toxin A had a clinical response based on the Patient Global Impression of Improvement. The median duration of their responses was 373 days, significantly longer than the 62 days or less for placebo (p <0.0001). In the botulinum toxin A group increased post-void residual urine (12 of 28 women or 43%) and urinary tract infection in those with increased post-void residual urine (9 of 12 or 75%) exceeded expected ranges. Further injections were stopped after 43 patients were randomized, including 28 to botulinum toxin A and 15 to placebo. Conclusions: Local injection of 200 U botulinum toxin A was an effective and durable treatment for refractory overactive bladder. However, a transient post-void residual urine increase was experienced in 43% of patients. Botulinum toxin A for idiopathic overactive bladder is still under investigation. © 2008 American Urological Association.