Associations between headache, including migraine, and gastrointestinal (GI) symptoms were studied in a large questionnaire-based cross-sectional study (the Head-HUNT Study). The headache questionnaire was completed by 43 782 individuals, who answered all the questions concerning nausea, reflux symptoms, diarrhoea and constipation. In the multivariate analyses, adjusting for age, sex, educational level, medication use, depression and anxiety, a higher prevalence of headache was found in individuals with much reflux [odds ratio (OR) 2.4, 95% confidence interval (CI) 2.2, 2.6], diarrhoea (OR 2.4, 95% CI 2.1, 2.8), constipation (OR 2.1, 95% CI 1.9, 2.4) and nausea (OR 3.2, 95% CI 2.6, 3.8) compared with those without such complaints. All the GI symptoms investigated seemed to be approximately as common among persons with non-migrainous headache as among migraine sufferers, but the association between headache and GI complaints increased markedly with increasing headache frequency. This may suggest that headache sufferers generally are predisposed to GI complaints.