This study aimed to assess the resource utilisation, health benefits and cost-effectiveness of intra-detrusor injections of botulinum neurotoxin-A (BoNT/A) in patients with overactive bladder (OAB). 101 patients with urodynamically-proven detrusor overactivity of either neurogenic (NDO; n = 63) or idiopathic (IDO; n = 38) origin received intra-detrusor injections of 200-300 units of BoNT/A in 20-30 ml saline as part of a research protocol. Twenty-nine patients received repeat injections after 7-26 months. Symptom severity and urodynamic parameters were assessed at 0, 4 and 16 weeks. The cost of therapy was quantified based on the NHS resources used by typical patients and was used to calculate the cost-effectiveness of BoNT/A compared with standard care from the perspective of the UK NHS. In an intent-to-treat analysis, 82% of patients showed a 25% or greater improvement in at least two out of five parameters (urinary frequency, urgency, urgency incontinence episodes, maximum cystometric capacity and maximum detrusor pressure) four weeks after treatment, reducing to 65% after 16 weeks. A 50% or greater improvement in the frequency of micturition, urgency or urgency incontinence was seen in 73% of patients at four weeks and 54% at 16 weeks. There were no significant differences between IDO and NDO patients in the proportion meeting these endpoints. Therapy cost pounds 826 per patient, with a cost-effectiveness ratio of pounds 617 per patient-year with > or = 25% clinical improvement. This study demonstrates that intra-detrusor BoNT/A is an effective treatment for OAB that is highly likely to be cost-effective in both idiopathic and neurogenic disease.