This study will be executed according to a randomized double-blind placebo-controlled trial with two parallel groups, treated over the period of 6 months with escitalopram or placebo.

Hypotheses: Escitalopram is more effective than placebo in the control of gastrointestinal symptoms, in irritable bowel syndrome (IBS) patients with panic disorder. Escitalopram is more effective than placebo in the control of psychiatric symptoms, in IBS patients with panic disorder.

Inclusion Criteria:

1. IBS will be diagnosed according to the Rome III criteria by a gastroenterologist.
2. Subtyping of IBS patients will be performed using the following classification according to the Rome III criteria: IBS with constipation (IBS-C), IBS with diarrhea (IBS-D), mixed IBS (IBS-M) or not subtyped IBS (IBS-U).
3. Based on the medical history and previous examination, no other causes for the abdominal complaints can be defined.
4. A panic disorder will be diagnosed based on DSM IV criteria by a psychiatrist.
5. Age above 18 years and under 70 years.
6. Given written informed consent.

Exclusion Criteria:

1. Inability to stop medication that can influence gastrointestinal motility or perception (like loperamide, butylscopolamine, duspatalin, metoclopramide, domperidon, erytromycin), serotonin metabolism (like carbidopa, food supplementation), or epigenetics (like valproic acid), or containing perforated St. John's wort (Hypericum perforatum). In general this medication can be stopped without problems. However, this can increase symptoms. When the increase in symptoms will be to high, this medication will be restarted and the patient excluded form the study.
2. Administration of investigational drugs in the 180 days prior to the study.
3. Major abdominal surgery interfering with gastrointestinal function (uncomplicated appendectomy, cholecystectomy and hysterectomy allowed, and other surgery upon judgement of the principle investigator), epilepsy or (hypo)manic episodes.
4. Pregnancy and lactation.
5. Excessive alcohol consumption (> 20 alcoholic consumptions per week) or drug abuse.
6. Co-intervention or other treatment for IBS or anxiety, with the exception of initial co-intervention with benzodiazepines (alprazolam) contrasting side effects due to SSRI's during the first two weeks of dose elevation.
7. Known prolongation of QT-interval or long-QT-syndrome, other cardiac disease, or use of medication with known prolongation of QT-interval.