The aim of this trial was to examine the effects after 2 years of a single intravesical botulinum toxin-A injection (BTX-A). This prospective, observational study was conducted using urodynamic measurements and quality of life (QoL) assessment to document the effect after 2 years of a single 100 I.U. injection of BTX-A into the vesical detrusor muscle. Twenty-six patients were followed up for 2 years after a first intravesical BTX-A injection. Of these 26 patients, one was a primary failure, three were lost to follow-up, and 11 patients had a repeated injection at 5-26 months (one patient had a third injection). Seven of the remaining 11 patients in the single injection group were recommended repeated injection or another treatment, and four required no other treatment. In conclusion, 2 years after a single BTX-A injection statistically significant differences in urodynamics and an improvement in QoL could still be demonstrated.