To evaluate the effectiveness of dimethyl sulfoxide in the treatment of patients with biopsies suggestive of interstitial cystitis, 33 patients underwent a controlled crossover trial. Patients were allocated randomly to receive 50 per cent dimethyl sulfoxide or placebo (saline). The medication was administered intravesically every 2 weeks for 2 sessions of 4 treatments each. Response was assessed urodynamically and symptomatically. Thirty women and 3 men (mean age 48 years and mean duration of symptoms 5.5 years) were entered into the study. No significant side effects to dimethyl sulfoxide were noted. When assessed subjectively, 53 per cent of dimethyl sulfoxide treated patients were markedly improved compared to 18 per cent of the placebo treated patients. Of the dimethyl sulfoxide group 93 per cent had objective improvement versus 35 per cent of the placebo group. Thus, dimethyl sulfoxide proved to be superior to placebo in the objective and subjective improvement of patients with interstitial cystitis.