Most drugs are ineffective for the long-term treatment of irritable bowel syndrome (IBS). The beneficial effects of medical treatment of IBS are poor and last for only a relative short time. Over a period of 6 months, we investigated the effectiveness of cimetropium bromide, a new antimuscarinic compound, in patients with IBS. Forty-eight patients were treated at random and in double-blind fashion with cimetropium bromide (50 mg, tid) or placebo for 6 months. Personal diary cards and monthly check-ups guaranteed the monitoring of symptoms (mainly pain). In addition, personality patterns (MHQ-CBA tests) were obtained for the patients before and after therapy, both to detect possible psychoneurotic traits and to observe the changes in these traits in relation to the changes in pain symptoms. Three patients on placebo and one on cimetropium dropped out. At the end of therapy, pain scores had decreased an average of 16% in the placebo group and 87% in the cimetropium group (p less than 0.01). Twenty patients (87%) on cimetropium versus five patients (24%) on placebo considered themselves to be globally improved (p less than 0.01). The MHQ test showed significant improvement in the anxiety score in the cimetropium group only. The CBA test confirmed a significant decrease in anxiety state (STAI-X-1) after cimetropium treatment. Eleven patients (48%) on cimetropium reported side effects (mainly dry mouth and sleepiness), but none withdrew from the study. The results of this trial indicate that long-term treatment of IBS with cimetropium bromide significantly improves symptoms and associated psychological disorders.