INTRODUCTION AND HYPOTHESIS:For decades, intravesical dimethyl sulfoxide (DMSO) cocktail therapy has been used for the treatment of interstitial cystitis/bladder pain syndrome (IC/BPS), but little is known about its long-term efficacy. We aimed to assess the long-term efficacy of intravesical DMSO/heparin/hydrocortisone/bupivacaine therapy in patients with IC/BPS. METHODS:Patients with IC/BPS from our institutions who underwent this therapy with >2 years follow-up were surveyed with O'Leary-Sant interstitial cystitis symptom and problem index questionnaires before and after therapy. Chart reviews and telephone surveys were then conducted to determine their posttherapy course. RESULTS:Of 68 eligible women, 55 (80.0%) with a median follow-up of 60 months (range 24-142) were surveyed. Their mean age at therapy onset was 44.8 years and their mean body mass index was 26.2 kg/m2. There were statistically significant improvements in O'Leary-Sant and pain scores of 23-47% at both 6 weeks and the end of the follow-up period. At the end of the follow-up period, 19 of the 55 women (34.5%) were cured (requiring no further treatment) and 12 (21.8%) were significantly improved (requiring only ongoing oral medication). Univariate and multivariate analyses showed that DMSO treatment failure was more likely in patients with pretreatment day-time urinary frequency more than 15 episodes per day (OR 1.41), nocturia more than two episodes per night (OR 2.47), maximum bladder diary voided volume <200 ml (OR 1.39) and bladder capacity under anaesthesia <500 ml (OR 1.6). CONCLUSIONS:At a median follow-up of 60 months, intravesical DMSO cocktail therapy appeared moderately effective for the treatment of IC/BPS. Treatment failure was more frequent in patients with pretreatment symptoms of reduced bladder capacity.