PURPOSE: Patients with multiple sclerosis often experience overactive bladder symptoms. High dose intradetrusor botulinum toxin A treatment is effective but often results in urinary retention and urinary diversion via a catheter. In this pilot study we evaluated whether only 100 U botulinum toxin A would significantly decrease overactive bladder symptoms in patients with multiple sclerosis without impairing pretreatment voluntary voiding. MATERIALS AND METHODS: Included in our study were 12 patients with multiple sclerosis who had overactive bladder symptoms such as urgency, frequency and/or urgency incontinence. The treatment effect was evaluated using data on 3 consecutive visits, that is before, and a mean +/- SD of 46.2 +/- 11.9 and 101 +/- 21 days after intradetrusor injection of 100 U Botox, including the results of cystometry and uroflowmetry at visits 1 and 2, and uroflowmetry alone at visit 3. Patients completed a 3-day voiding diary for all 3 visits. RESULTS: Maximum bladder capacity significantly increased and maximum detrusor pressure decreased. Daytime and nighttime frequency, urgency and pad use significantly decreased. Post-void residual volume significantly increased initially but decreased until 12 weeks. Median time to re-injection due to recurrent overactive bladder symptoms was 8 months. CONCLUSIONS: Overactive bladder treatment in patients with multiple sclerosis using 100 U Botox intradetrusor injections seems to be effective and safe. Despite slightly impaired detrusor contractility most patients still voided voluntarily without symptoms. Thus, 100 U Botox may be a reasonable treatment option for overactive bladder symptoms in patients with multiple sclerosis who still void voluntarily.