Painful bladder disease, sensory bladder disease, chronic abacterial cystitis and interstitial cystitis are ill-defined conditions of unknown etiology and pathogenesis, and, therefore, they are without any rational therapy. Pathogenetic theories concerning defects in the epithelium and/or mucous surface coat (including glycosaminoglycans) of the bladder, and theories concerning immunological disturbances predominate. Sodium pentosanpolysulfate (Elmiron) acts by substituting a defective glycosaminoglycan layer and inhibits complement reactions in inflammatory processes. We compared sodium pentosanpolysulfate versus placebo in a prospective double-blind, clinically controlled multicenter trial of 115 patients with painful bladder disease. Two protocols were used. Protocol A included 43 patients with clinically and pathologically anatomically verified interstitial cystitis (28 or more mast cells per mm.2), and protocol B included 72 patients with a painful bladder and unspecific histological findings. The patients were randomized to receive either sodium pentosanpolysulfate (200 mg. twice daily) or placebo capsules for 4 months. Before and after the trial the patients were evaluated with symptom grading, urodynamics and cystoscopy with distension and deep bladder biopsies. The results showed no difference between the pre-trial and post-trial values in the sodium pentosanpolysulfate and placebo groups in both protocols in regard to symptoms, urodynamic parameters, cystoscopic appearance and mast cell counts. A significant increase in the cystoscopically determined bladder capacity in the sodium pentosanpolysulfate group in protocol A was found. We conclude that no statistically or clinically significant effect of sodium pentosanpolysulfate was found compared to placebo in patients with painful bladder disease.