Overactive bladder (OAB) is a highly prevalent disorder in the pediatric population. This condition comprises many urinary symptoms, such as urgency, increased daytime frequency of micturition, urge incontinence and nocturia1, 2. These symptoms are especially troublesome for the pediatric patients and their family since it causes embarrassment and it limits everyday activities and impairs children's development. Furthermore, serious complications are seen if this condition is not treated properly, as urinary tract infection, vesico-ureteral reflux and dysfunctional voiding. Antimuscarinic agents are the current pharmacologic mainstay for OAB. Many side effects are reported with the clinical use of antimuscarinics. In the last years, Fesoterodine, a new antimuscarinic, has been developed for the treatment of OAB. Studies show significant improvement of clinical symptoms in adults with OAB and fewer side effects. The outcomes for the pediatric population remain unknown due to lack of studies. Antimuscarinic agents are the mainstay of the current treatment of OAB. Oxybutynin is the most widely antimuscarinic agent used in the pediatric population and is the only molecule approved by Health Canada for children with OAB. However, some patients have a suboptimal response to antimuscarinic and many experience side effects. Children with OAB therefore represent a disease population with a need for an alternative effective, safe and well-tolerated therapy to help manage the overactive detrusor, reducing or preventing incontinence. Fesoterodine is a new antimuscarinic drug available as a prolonged release (PR) tablet formulation, and is approved in Europe, Canada and the USA at doses of 4 mg and 8 mg once daily for the treatment of OAB in adults; it is not approved for use in the pediatric population. Before randomization, subjects will undergo 2 weeks of urotherapy. At the end of these 2 weeks, the inclusion and exclusion criteria will be reassessed and the subjects admissible for the study will be randomized for a 8 week-treatment period during which the urotherapy will be continued. Eligible subjects will be randomized to 8 weeks of single-blind treatment with Fesoterodine 4 mg Po Die or Oxybutynin XL 10 mg Po Die. At the end of the 8 week-treatment period, subjects will stop their current therapy for one week. At week 9, both cohorts will do a cross over. The cohort on Fesoterodine will be on Oxybutynin XL and vice versa. After 4 weeks on any given medication, the possibility of up-titration will be assessed. On a telephone interview with the research nurse, patients and parents will be questioned on compliance, tolerability and efficacy. If the patient is taking the medication ≥80% of the time, does not have any significant side effects and still has significant OAB symptoms, the investigators will offer a dose increase (Fesoterodine 8mg or Oxybutynin XL 20mg daily). If accepted, the medication will be provided with instructions to report any new side effects. Oxybutynin XL (Ditropan XL) is the extended release and once a day formulation of Oxybutynin (actual gold standard for OAB in children). By using this formulation with children who can swallow pills, the control group has the same once a day regimen as Fesoterodine (Toviaz), thus avoiding this possible bias. The bid or tid immediate release formulation is known to create more side effects and decreases the compliance (sometime hard to administer the afternoon dose to children at school).

Subjects will complete a 3-day voiding diary prior to each medical visit to assess the efficacy of the single-blind treatment and urotherapy. Visits will be done on week -2, 0, 8 and 17 (+/- 5 days)