To investigate diabetic alterations of small intestinal transit and bacterial growth, we performed hydrogen breath tests (10 g lactulose via duodenal tube at the ligament of Treitz), bacterial cultures, and determinations of unconjugated serum bile acids in 19 patients with long-standing diabetes and 7 healthy controls. Asymptomatic diabetics had a late rise in breath hydrogen, indicating prolonged jejunal-cecal transit (86 +/- 10 min, p less than 0.05) as an early pathogenic event. Rise in breath hydrogen in symptomatic diabetics (constipation: 50 +/- 6 min; diarrhea: 41 +/- 11 min) was not significantly different from controls (57 +/- 8 min). Bacterial studies and increased unconjugated serum bile acids suggest bacterial overgrowth in some symptomatic diabetics. Bacterial overgrowth was associated more frequently (p less than 0.05) with a rise in breath hydrogen before 45 min or after 75 min. Changes in the hydrogen breath test, bacterial growth, or unconjugated serum bile acids did not correlate with gastrointestinal symptoms of diabetes.