Objectives.To evaluate further the intravesical potassium sensitivity test (PST) as an indicator of the epithelial leak of interstitial cystitis (IC) and determine whether successful pentosan polysulfate (PPS; Elmiron) treatment is associated with a change in PST results. Most individuals with IC appear to have an abnormally permeable epithelium that allows urinary solutes such as potassium to penetrate to the bladder interstitium, provoking symptoms. Methods. Data were from an optimal dose trial of PPS in IC. Patients underwent a PST before and after a 32-week trial of 300, 600, or 900 mg PPS/day. The response to PPS treatment was measured using the Patient Overall Rating of Improvement in Symptoms scale. The before and after treatment PSTs and Patient Overall Rating of Improvement of Symptoms scores were compared. Results. Of 377 patients with IC at 28 centers, 302 (80%) had a positive PST at entry. Of the 198 patients who completed the study, 153 were PST positive at entry and 92 (60%) showed clinical improvement at exit. Clinically improved patients had significant improvement on the PST analog pain and urgency scales (3.2 to 1.3 and 3.6 to 1.9, respectively; P <0.0001). In contrast, patients with no clinical improvement had no significant change in pain (3.1 to 2.7) or urgency (3.6 to 3.2). Conclusions.PST shows abnormal epithelial permeability in most patients with IC and a significant reduction in this permeability after successful PPS therapy. PST appears to be a valid indicator of epithelial abnormality and a reliable test in the diagnosis of IC. Copyright © 2002 Elsevier Science Inc.