The pathogenesis of irritable bowel syndrome (IBS) has been related more to dysmotility of the colon than to abnormalities of the small intestine. To look for small bowel abnormalities, we recorded ultraluminal pressures in 16 patients with IBS. All patients complained of abdominal pain, and diarrhea (n = 8) or constipation (n = 8) were also prominent symptoms. Comparable studies were performed on 16 age-matched controls. The observations include diurnal and nocturnal fasting recordings and the response to a fatty meal. Periodicities of the interdigestive migrating myoelectric complexes were shorter in IBS (p less than 0.05); this was due to much shorter diurnal cycles in patients with diarrhea (77 +/- 10 min) than those with constipation (118 +/- 15 min) or controls (113 +/- 10 min, both p less than 0.05). All groups exhibited circadian changes, with nocturnal cycles being more frequent. Two specific patterns of small bowel motor activity were more common in IBS--ileal propulsive waves and clusters of jejunal pressure activity (both p less than 0.05 compared to controls). Moreover, cramping abdominal pain was usually noted in IBS when ileal motility was propulsive; jejunal bursts were also sometimes associated with abdominal symptoms. We conclude that motility of the small intestine is modified in some patients with IBS and that certain motor patterns are related to their symptoms.