Inclusion Criteria:

1. Age 65 or older
2. Diagnosis of idiopathic PD (ICD9=332.0)
3. MMSE 24, able to give informed consent and complete questionnaires and voiding diaries.

4. Urological work-up within 3 months of enrollment to:

   • Rule out treatable causes of urinary symptoms

     • Urinalysis (UA)
     • Post-void residual ultrasound (PVR)
     • Urinary cytology

   • Documented symptoms OAB on screening 3-day voiding diary:

     • Average of 1 urgency episode / 24 hours, and
     • Average of 8 micturitions / 24 hours
     • Subjective complaints of symptoms for 3 months

Exclusion Criteria:

1. Exposure to anticholinergics or antispasmodics within the last 4 weeks (among them: atropine, tolterodine, benztropine, trihexyphenidyl, dicyclomine, hyoscyamine, and scopolamine)
2. Exposure to drugs with known effects on cognition (i.e. opioids, benzodiazepines or sedating antihistamines) within the last week
3. Exposure to drugs contraindicated or cautioned in use with the 2 study medications (drugs that also use the cytochrome P450 enzyme, primarily CYP3A4). These include: ketoconazole, itraconazole, miconazole, erythromycin, clarithromycin, ritonavir, nelfinavir, nefazodone, flecainide, thioridazine and tricyclic antidepressants.
4. Nonpharmacological treatment of OAB within the last 4 weeks (for example: biofeedback, physical therapy, acupuncture)
5. Uncontrolled narrow angle glaucoma
6. History of gastric or urinary retention / dysmotility (ulcerative colitis, myasthenia gravis and severe constipation)
7. History of hepatic or renal impairment
8. History of severe gastro-esophageal reflux disease and/or use of bisphosphonates, patients at risk for esophagitis
9. Previous exposure to anticholinergic for OAB symptoms that resulted in side effects that caused cessation of the medication