PURPOSE: Open, observational studies of intradetrusor injections of botulinum toxin for detrusor overactivity have reported beneficial effects. We tested the efficacy and safety of botulinum toxin B for the treatment of the overactive bladder in a randomized, double-blind, placebo controlled crossover trial. MATERIALS AND METHODS: A total of 20 patients 18 to 80 years old with detrusor overactivity unresponsive to oral antimuscarinic agents participated in the study. They were injected with either placebo (20 ml normal saline) or botulinum toxin B (5,000 IU diluted up to 20 ml) intravesically in a day case setting. After 6 weeks the treatments were crossed over without washout in line with previous findings. The primary outcome was the paired difference in change in average voided volumes. Frequency, incontinence episodes and paired differences in quality of life measured by the King's Health Questionnaire were the secondary outcome measures. RESULTS: The Wilcoxon signed ranks test was used to test the paired difference in change between treatment phases. Little carryover was noted in the second arm placebo and the placebo data from both arms were included in analysis. There were clinically statistically significant paired differences in the change in average voided volume, urinary frequency and episodes of incontinence between active treatment and placebo (average voided volume: 95% CI difference 16, 122; Z2.5; p = 0.012/weekly frequency: 95% CI -21, -1; Z2.1, p=0.033/weekly incontinence: 95% CI -26, -7; Z3.3; p = 0.001). There were similarly significant paired differences in the change in quality of life affecting 5 domains of the King's Health Questionnaire. CONCLUSIONS: This double-blind, placebo controlled, crossover study provides evidence of the efficacy of botulinum toxin B in the treatment of overactive bladder. Autonomic side effects were observed in 4 patients. The short duration of action will presumably limit the use to patients who have experienced tachyphylaxis with botulinum toxin A.