We completed a phase 1/2 trial to evaluate the safety and potential efficacy of direct intravesical instillation of a botulinum type A toxin/dimethyl sulfoxide (DMSO) solution for treatment of idiopathic detrusor overactivity in women. Twenty-five women with medication-resistant, urodynamic-confirmed idiopathic detrusor overactivity were enrolled. A total of 9 patients were treated in phase 1 of the study. Three patients were given a 66% dosing of solution; 22 patients received the full 300 units of botulinum toxin and 50 mL of DMSO (50% concentration). Adverse effects, 24-hour pad weights, episodes of incontinence, postvoid residuals, and scores on the Blaivas-Groutz anti-incontinence scale, Indevus Urgency Severity Scale, Incontinence Impact Questionnaire-short form, and Urogenital Distress Inventory (6 items) questionnaire were recorded at baseline, 1 month, and 3 months after instillation. No serious adverse effects or clinically important increases in postvoid residual occurred. Among the 21 women who completed phase 2 of the study, the median number of incontinent episodes decreased from 4 at baseline to 2 at 1 month (P=.004) and increased to 4 at 3 months (P=.81). Median scores improved from baseline to 1 month on the Incontinence Impact Questionnaire (from 13 to 7; P=.007) and Urogenital Distress Inventory (from 10 to 5; P=.003). Although 11 women (52%) reported severe urgency based on the Indevus Urgency Severity Scale at baseline, only 1 (5%; P<.001) and 3 (14%; P=.004) women reported severe scores at 1 and 3 months, respectively. Direct instillation of botulinum toxin/DMSO solution is safe. Its promising clinical effect warrants further evaluation in a randomized, placebo-controlled phase 3 setting.