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ABSTRACT INTRODUCTION: Irritable bowel syndrome (IBS) is the most common disorder of gasterointestinal tract. The prevalence of IBS is between 14% to 22%. This study was aimed to demonstrate the relationship between post nasal discharge (PND) and IBS. MATERIALS & METHODS: This was a case- control study. The case group including 67 patients with IBS and no history of psychologic or chronic disorders, refered to a clinic of internal diseases, were compared with 75 patients without IBS selected by paired matching method. RESULTS: Of patients with IBS, 59.7% (40 cases) were male and 40.3% (27 cases) were female. Mean age of the patients was 27.9 years (SD= 8.73). In 41.8% of the patients (28 cases) evidences of sinusitis were found in radiography of paranasal sinuses. Sinusitis had a signtificant association with IBS (P=0.01, OR= 4.27, CI= 1.19- 16.74, 2= 5.37). PND was also significantly associate with IBS (P= 0.001, CI= 2.99- 13.99, 2=18.69). CONCLUSIONS: Our Study confirmed the realationship between upper respiratory tract infections, particulaly sinusitis, and IBS. This may suggest that IBS is a systemic disorder involving multiple organ systems.