BACKGROUND: An unexplained epidemic of obesity is occurring but the relationship between obesity and gastrointestinal (GI) tract function is unclear. We aimed to evaluate the association between body mass index (BMI) and specific GI symptoms. METHODS: A birth cohort in Dunedin, New Zealand, aged 26 yr (n = 980, 94% of total original sample) was evaluated using a validated GI symptom questionnaire. Categories of GI symptom complexes were defined a priori. The association of reported GI symptoms with BMI (kg/m2) was assessed adjusting for gender. RESULTS: The prevalence of obesity (BMI ≥ 30 kg/m2) was 12%; 30% were overweight. There was a significant univariate positive association between increased BMI and diarrhea (>3 stools/day, loose stools, or urgency); the sex adjusted odds ratio for obese versus normal weight was 1.8 (95% CI 1.1, 2.9; p = 0.02). Abdominal pain associated with nausea or vomiting was positively associated with increased BMI (OR 2.0, 95% CI 1.0, 2.9; p = 0.04). Being overweight was negatively associated with abdominal pain and constipation (OR 0.4, 95% CI 0.2, 0.9; p = 0.02). Irritable bowel syndrome and reflux symptoms were not significantly associated with increased BMI. Waist-to-hip ratios were not significantly associated with GI symptoms. No study members were taking antiobesity medications. Hemoglobin A1c levels were not associated with any of the GI symptoms. CONCLUSIONS: In a general population sample of young adults, increasing BMI was associated with diarrhea and abdominal pain with nausea/vomiting.