Summary— Interstitial cystitis is rarely considered as a cause of urinary symptoms in referrals to gynaecology clinics. Recent concepts in the diagnosis of this condition mean that it is emerging as a much more common entity, with both early and late forms of the disease being described. Mast cell density in the detrusor muscle has been reported to be useful as a disease marker to substantiate the diagnosis of interstitial cystitis where no classical diagnostic features exist. We assessed mast cell counts in bladder biopsies from 27 women with idiopathic sensory urgency and 10 control patients about to undergo a colposuspension procedure for pure genuine stress incontinence; 30% of the study group had a clear increase in the detrusor muscle mast cell population (detrusor mastocytosis). No control patient showed such an increase. Early interstitial cystitis should be considered as a possible cause of lower urinary tract symptoms in patients with apparently idiopathic sensory urgency.