Irritable bowel syndrome (IBS) is the most common gastrointestinal disorder encountered in general practice. Although enormous progress in the pathogenesis of IBS has been made, it is still poorly understood. A significant proportion of patients with IBS, report onset of symptoms following an episode of acute gastroenteritis. Aim of the study was to determine whether post infective irritable bowel syndrome (PI-IBS) has histological or clinical features that are distinct from those of IBS patients with no history of preceding infection. A total of 50 consecutive IBS out patients and 10 healthy control subjects completed a questionnaire detailing symptoms, mode of onset, and previous psychiatry history. All underwent a full diagnostic workup including colonic biopsy and histopathology. Patients were divided according to onset of symptoms into post infectious IBS (PI-IBS, N=18) and non-post infectious IBS (non-PI-IBS, N=32) patients. Diarrhea predominantly occurred more frequently in PI- IBS (70%) than in non-PI-IBS (40%) patients (p<0.05). Histopathological examination of the biopsy specimen from the sigmoid colon of the observed individuals showed a significant increase in the number of chronic inflammatory cells in both groups of IBS patients comparing to the control group (p<0.001). Lymphoid follicles were increased in both groups of IBS patients in comparison to control group (p<0.05). Mast cells were increased in IBS patients comparing to the control group (p<0.05) and it was significantly increased in the PI-IBS patients than the non-PI-IBS patients (p<0.001). Individuals with PI-IBS are clinically distinct subgroup characterized by predominant diarrheal symptoms and increased mast cells compared to those with non PI-IBS.