The objective of the study was to determine whether intravesical pentosanpolysulfate (PPS) reduces symptoms associated with bladder pain syndrome/interstitial cystitis (BPS/IC). In a prospective, uncontrolled, open-label study, 29 female patients with BPS/IC received 300 mg PPS intravesically twice a week for 10 weeks and thereafter a voluntary maintenance therapy once a month. Treatment response was assessed by Visual Analog Scale (VAS) for quality of life and O'Leary-Sant Symptom and Problem Index (OSPI). Patients were tested before treatment, after 5 weeks of treatment, and 1 week, 3, 6, and 12 months after termination of the initial treatment. Twenty-five patients underwent the 10-week treatment and the 3-month follow-up. Mean reduction of VAS/OSPI was from 8.8/26.4 before to 4/15.3 after treatment, 3.8/15.2 after 3 months, 3.8/14 after 6 months, and 3.4/12.1 after 12 months. In 21 patients, renewed instillation or maintenance therapy was necessary. Intravesical treatment with PPS reduces both the VAS and the OSPI in patients with BPS/IC.