PURPOSE: To determine the efficacy of intravesical hyaluronic acid (HA) instillation in treating patients with refractory interstitial cystitis/painful bladder syndrome (IC/PBS) and to identify any related factors that influence its therapeutic effect. METHODS: Thirty-three female IC/PBS patients who demonstrated poor or unsatisfactory responses to previous treatments between December 2010 and October 2012 were enrolled. Despite previous treatments, the enrolled patients had visual analogue scale (VAS) pain scores > or =4 and total scores (symptom and bother scores) > or =13 on the pelvic pain and urgency/frequency (PUF) questionnaire and > or =12 on the O'Leary-Sant interstitial cystitis symptoms index (ICSI)/problems index (ICPI). All patients received once weekly intravesical instillations of 40-mg HA diluted in 50-mL saline for 4 weeks. The efficacy of the HA instillation was evaluated by comparing the mean changes in the scores of the VAS and questionnaires from baseline to 4 weeks after treatment. Improvement was defined as a > or =2 decrease in the VAS. Moreover, we investigated the effects of the presence of Hunner's ulcer and previous treatment modalities on the therapeutic outcome of HA instillation. RESULTS: The mean age was 57.0+/-1.8 years (range, 28-75 years). The VAS score significantly decreased from baseline to 4 weeks after treatment (-2.5, P<0.001). The mean changes in the PUF, ICSI, and ICPI from baseline to 4 weeks after the treatment were -3.8 (P<0.001), -2.3 (P<0.001), and -2.7 (P<0.001), respectively. Twenty patients (61%) showed improvements. Previous treatment modalities did not affect the efficacy of HA instillation and the presence of Hunner's ulcer was unrelated to outcomes. No complications were observed. CONCLUSIONS: These results show that intravesical HA instillation is an effective and safe treatment for patients with refractory IC/PBS. Previous treatment modalities and presence of Hunner's ulcer do not affect the efficacy of HA instillation.