In a prospective, randomised, double-blind, placebo-controlled study 40 women with sensory urgency or detrusor instability randomly received either magnesium hydroxide (group A) or placebo (group B). Pre- and post-treatment symptoms, frequency-volume charts and cystometry results were compared. Eleven of 20 patients receiving magnesium (55%) reported a subjective improvement of their urinary symptoms, compared with five patients taking placebo (20%). In both study groups there was no statistically significant difference in pre- and post-treatment urodynamic parameters in those reporting symptomatic improvement. Magnesium was well tolerated by patients in group A, and no side effects were reported. These results suggest that magnesium hydroxide may be beneficial for detrusor instability or sensory urgency in women.