This trial compares the change in urgency urinary incontinence episodes over 6. months, tolerability and cost effectiveness between women receiving daily anticholinergic therapy plus a single intra-detrusor injection of saline versus a single intra-detrusor injection of 100. U of botulinum toxin A plus daily oral placebo tablets. We present the rationale and design of a randomized-controlled trial, Anticholinergic versus Botulinum Toxin, Comparison Trial for the Treatment of Bothersome Urge Urinary Incontinence: ABC trial, conducted by the NICHD-funded Pelvic Floor Disorders Network. We discuss the innovative nature of this trial and the challenges related to choice of patient population, maintaining masking, cost effectiveness, ethical considerations, measuring adherence, and placebo development and testing. Enrollment began in April, 2010. 242 participants will be randomized and primary outcome data analysis is anticipated to begin in mid 2012. Several challenges in the trial design are discussed. Randomization to placebo intra-detrusor injections may limit recruitment, potentially impacting generalizability. Other challenges included the heavy marketing of drugs for overactive bladder which could impact recruitment of drug-naïve women. In addition, anticholinergic medications often cause dry mouth, making masking difficult. Finally, adverse reporting of transient urinary retention is challenging as there is no standardized definition; yet this is the most common adverse event following intra-detrusor botulinum toxin injection. The ABC trial will help women with urgency urinary incontinence balance efficacy, side effects and cost of anticholinergic medication versus botulinum toxin intra-detrusor injection. The results have the potential to fundamentally change the therapeutic approach to this condition. © 2011 Elsevier Inc.