OBJECTIVES To assess, in a prospective study, whether botulinum toxin A (BTXA) injected into the detrusor muscle is safe and well tolerated, produces significant changes in urodynamic variables, if the effect is durable, as measured by the Bristol Female Lower Urinary Tract Symptom Questionnaire (BFLUTS), and to assess changes in quality of life, as measured by the Kings Health Questionnaire (KHQ), when it is used to manage idiopathic detrusor overactivity. PATIENTS AND METHODS This was a single‐centre, prospective unrandomized study of 15 women to assess the efficacy of a single dose of 300 units of BTXA injected intravesically into the detrusor muscle, under cystoscopic control. Patient evaluation included a full history and examination, frequency/volume charts, BFLUTS and KHQ scores, and a conventional urodynamic study at baseline and at 6 weeks after treatment. Symptomatic improvement was assessed at 6 weeks and every 4 weeks thereafter until baseline values were reached. RESULTS All 15 patients completed the study; 14 noted an improvement in urgency and frequency immediately after treatment. There were no major adverse effects reported to date. The volume at first desire to void increased in 13 patients (P < 0.006), the maximum cystometric capacity increased in 10 (P < 0.011) and six of the 15 had no evidence of detrusor overactivity; in the remaining eight the volume at first overactive contraction increased in six (P < 0.0023) and the volume at first overactivity incontinence increased in 11 (P < 0.005). The median modified projected isovolumetric pressure decreased significantly (P = 0.01), from 69 to 45. The improvement in frequency appeared to last up to 24 weeks, and that in urinary incontinence up to 20 weeks. The overall improvement in quality of life was maintained up to 24 weeks. CONCLUSION BTXA appears to be a safe treatment with good clinical efficacy in refractory idiopathic detrusor overactivity; the effects seem to last 20–24 weeks.