A randomized, prospective, double-blind, placebo-controlled study was conducted at 7 clinical centers on 148 patients. Patients received orally either 100 mg. pentosanpolysulfate (a synthetic polysaccharide) 3 times per day or a placebo. Of the patients on drug therapy 32% showed significant improvement compared to 16% of those on placebo (p = 0.01). This study provides a model to assess this disease quantitatively in a prospective manner using a method whereby the patients globally assess their symptoms as either worse or improved by 0, 25, 50, 75 or 100%. Patients on drug therapy also experienced a significant decrease in pain and urgency (p = 0.04 and 0.01) on analogue scales when compared to placebo and also more drug patients showed an average increase of more than 20 ml. in voided volume than did placebo patients (p = 0.02). All adverse effects were minor, with 7 in the drug group and 10 in the placebo group. The results support the concept that some patients with the interstitial cystitis syndrome may have abnormal bladder surface glycosaminoglycans.