An intravesical instillation of 100 ml 1 or 2 mmol/l capsaicin has been used to treat detrusor hyperreflexia giving rise to intractable urinary incontinence in 12 patients with spinal cord disease and two other patients with detrusor overactivity of non-spinal origin. Nine patients, all of whom had spinal cord disease, showed some improvement in bladder function. The benefit was only shortlived and partial in four, but the remaining five achieved complete continence while performing intermittent self catheterisation. Urodynamic studies in these nine patients showed an increase in mean (SD) bladder capacity from 106 (57) to 302 (212) ml and a fall in the maximum detrusor pressure from 54 (20) to 36 (10) cm of water. There were no short term ill effects from the instillation and the improvement in bladder function lasted for between three weeks to six months, when in some patients it was repeated. The improvement in bladder behaviour shown in this study can be interpreted as showing that capsaicin sensitive afferents play an important part in the pathogenesis of detrusor hyperreflexia in spinal humans. Intravesical capsaicin seems a promising means of treating intractable detrusor hyperreflexia and studies with this substance may shed new light on other disorders of detrusor activity that cause incontinence.