Aim: To determine how clusters (groups) of patients with respect to symptoms compare with a clinical diagnosis in patients with irritable bowel syndrome and non-ulcer dyspepsia. Methods: All patients who attended a gastroenterology practice at Nepean Hospital were included in the study. All patients received the previously validated Bowel Disease Questionnaire, and were independently assessed by the gastroenterologist. Factor analysis and a k-means cluster analysis were completed. Results: The study population comprised 897 patients [320 males (36%) and 577 females (64%)]. Factor analysis identified nine symptom factors: (1) diarrhoea; (2) constipation; (3) dysmotility; (4) dyspepsia/reflux; (5) nausea/vomiting; (6) bowel; (7) meal-related pain; (8) weight loss; and (9) abdominal pain. A k-means cluster analysis identified seven distinct subject groups, which included an undifferentiated group: (1) diarrhoea; (2) meal-related pain; (3) abdominal pain; (4) faecal indicators: (5) nausea/vomiting/weight loss; and (6) constipation. The majority of irritable bowel syndrome patients fitted into two cluster groups [diarrhoea (25%) and constipation (20%)], whereas those with non-ulcer dyspepsia predominantly fitted into the undifferentiated cluster (34%) and the nausea/vomiting cluster (18%). Conclusion: This study supports the concept of symptom subgroups, including the subdivision of patients into diarrhoea- and constipation-predominant irritable bowel syndrome.