Subgroups of patients with irritable bowel syndrome (IBS) are likely to respond differently to existing and evolving therapies. The following criteria for subgrouping may be considered: (1) Patients with different predominant bowel habits respond differently to treatment (antidepressants, 5HT3-antagonists, psychotherapy). (2) Postprandial exacerbation of pain or other gastrointestinal symptoms is seen in approximately half of patients with IBS and may identify patients who are more responsive to some classes of drugs (e.g., those targeted at motility). (3) Women appear to respond differently from men to 5HT3-antagonists, and there may be gender differences in gastrointestinal physiology. (4) There is more overlap in the diagnosis of functional dyspepsia and IBS than would be predicted by chance, and both are associated with hyperalgesia to intraluminal distention.